Continuously grating-tuned external cavity laser with automatic suppression of source spontaneous emission and amplified spontaneous emission

This application claims the benefit of provisional applications Ser. No. 60/191,699 filed Mar. 23, 2000 and No. 60/191,694 filed Mar. 23, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to grating-tuned external cavity lasers and more particularly to a method and apparatus for generating a continuously-tunable, low-noise laser beam in a grating-tuned external cavity laser.

2. Description of Related Art

Grating-tuned external cavity lasers produce continuously-tunable laser beams consisting of light with high coherence and very narrow linewidth. To obtain high coherence and narrow linewidth, a grating is generally employed to disperse the emission from a light source or gain element, and feed it back to the gain medium at a wavelength selected by a tuning device. Tunable laser beams can be produced either by rotating a grating in a Littrow-type arrangement, or a reflector in a Littman-type configuration. Littman-type tunable laser systems are described in the publications, “Spectrally Narrow Pulse Dye Laser Without Beam Expander,” by Michael G. Littman and Harold J. Metcalf, <i>Applied Optics</i>, Vol. 17, No. 14, pages 2224-2227, Jul. 15, 1978, and “Narrowband Operation Of A Pulsed Dye Laser Without Intracavity Beam Expansion” by l. Shoshan, N. N. Dannon, and U. P. Oppenheim, <i>Journal of Applied Physics</i>, Vol. 48, pages 4495-4497, 1977. A single-longitudinal-mode (very narrow linewidth) frequency tunable pulsed dye laser was described in the publication, “Single-Mode Pulsed Tunable Dye Laser,” by M. G. Littman, Optics Letters, Vol. 23, pages 138-140, 1978. This single-longitudinal mode laser provides a foundation for producing tunable narrow-bandwidth lasers.

FIG. 1 shows a prior art grating-tuned external cavity laser capable of producing a laser beam which is tunable over a broad range of wavelengths by rotation of a tuning reflector. Laser system <b>100</b> comprises pivot <b>102</b>, base <b>104</b>, plane reflector <b>106</b>, gain medium <b>108</b>, diffraction grating <b>110</b>, tuning reflector <b>112</b>, rotatable unit <b>114</b>, output laser beam <b>116</b> and first-order diffracted radiation <b>118</b>.

In the prior art system of FIG. 1, a proximal end of rotatable unit <b>114</b> is pivotably connected to base <b>104</b> by pivot <b>102</b>. Tuning reflector <b>112</b> is mounted on rotatable unit <b>114</b> forming an acute angle with respect to diffraction grating <b>110</b>, which is mounted on an upper surface of base <b>104</b>. Plane reflector <b>106</b> and gain medium <b>108</b> are mounted on base <b>104</b> and are disposed to produce a laser beam which is incident on diffraction grating <b>110</b> at a grazing angle, thereby generating output laser beam <b>116</b> and first-order diffracted radiation <b>118</b>.

In operation, rotating arm <b>114</b> pivots around pivot <b>102</b> such that tuning reflector <b>112</b> moves relative to diffraction grating <b>110</b>. Plane reflector <b>106</b> and gain element <b>108</b> generate a laser beam which is incident on diffraction grating <b>110</b> at a grazing angle. Part of this laser beam is reflected as output laser beam <b>116</b> and exits laser system <b>100</b>. The rest of the laser beam incident on diffraction grating <b>110</b> is diffracted and reflected to generate a light radiation pattern which includes first-order diffracted radiation <b>118</b>. First-order diffracted radiation <b>118</b> retro-reflects off tuning reflector <b>112</b> and is again incident on diffraction grating <b>110</b>.

Upon further diffraction and reflection by diffraction grating <b>110</b>, a portion of first-order diffracted radiation <b>118</b> enters gain element <b>108</b> and plane reflector <b>106</b>, thereby forming an external feedback laser cavity for laser system <b>100</b>. The wavelength of output laser beam <b>116</b> depends on the angle formed by grating surface <b>110</b> and the reflective surface of tuning reflector <b>112</b>, which may be adjusted by pivoting rotatable unit <b>114</b> around pivot <b>102</b>. Consequently, the wavelength of output laser beam <b>116</b> may be tuned by pivoting rotatable unit <b>114</b> around pivot <b>102</b>. Accurate positioning of pivot <b>102</b> enables mode-hop-free, continuous tuning of output laser beam <b>116</b> over the entire emission band of gain element <b>108</b>.

A laser system similar to the prior art system shown in FIG. 1 is described in the publication, “Novel Geometry for Single-Mode Scanning of Tunable Lasers,” by Michael G. Littman and Karen Liu, Optics Letters, Vol. 6, No.3, pages 117, 118, March, 1981. A mode-hop-free, Littman cavity laser system with broad-range tuning capabilities is set forth in the publication, “Synchronous Cavity Mode and Feedback Wavelength Scanning in Dye Laser Oscillators with Gratings,” by Harold J. Metcalf and Patrick McNicholl, <i>Applied Optics</i>, Vol. 24, No. 17, pages 2757-2761, Sep. 1, 1985. The publication “Scanning Geometry for Broadly Tunable Single-Mode Pulsed Dye Lasers,” by Guangzhi Z. Zhang and Kohzo Hakuta, <i>Optics Letters</i>, Vol. 17, No. 14, pages 997-999, Jul. 15, 1992, describes a dye laser system capable of continuously tuning a single-longitudinal-mode laser beam over a range of more than 190 cm⁻¹by employing a predefined rotation pivot for the tuning reflector and grating.

Various configurations of grating-tuned, Littman-type, external laser cavity systems capable of providing continuous, broadband, mode-hop-free laser beams have been disclosed in U.S. Pat. No. 5,319,668 to Luecke, U.S. Pat. No. 5,867,512 to Sacher, U.S. Pat. No. 5,771,252 to Lang, U.S. Pat. No. 5,802,085 to Lefevre, et al and the publication “Continuously Tunable Diode Lasers,” by Timothy Day, Frank Luecke, and Michel Brownell, <i>Lasers </i>&amp; <i>Optronics</i>, No. 6, June, 1993, pp. 15-17. According to these publications, accurate positioning of the pivot is paramount to obtain continuous, broadband tuning capability over the entire emission bandwidth of the gain medium.

Lowering the lasing threshold for grating-tuned external cavity lasers increases the laser power output in the presence of optical power loss occurring inside the laser cavity due to grating diffraction. A method for reducing power loss was described in the publication, “Lasing Threshold Reduction for Grating-Tuned Laser Cavities,” by Guangzhi Z. Zhang and Dennis Tokaryk, <i>Applied Optics</i>, vol. 36, No. 24, pages 5855-5858, Aug. 20, 1997. This publication introduced a laser system that utilized potentially wasted optical power in an effective feedback configuration.

Mode-hop-free, broadband tunable lasers have been extensively used in a wide range of applications, including laser spectroscopy, optical metrology, in-situ process monitoring and test and measurement of optical passive components in Dense Wavelength Division Multiplexing, Wavelength Division Multiplexing and optical fiber systems.

The output of grating-tuned, external cavity laser systems in the prior art generally consists of two spectral components: (1) a laser beam; and (2) background light radiation comprising Source Spontaneous Emission (“SSE”) and Amplified Spontaneous Emission (“ASE”) light radiation. The laser beam is the desired output component and consists of substantially coherent, narrow-linewidth laser light. The SSE and ASE radiation, however, constitutes an undesirable incoherent noise background which is emitted directly by the gain element.

The laser beam component of the laser output couples with the SSE and ASE background radiation component in space and time. Although the SSE and ASE background radiation is usually weak in power as compared to the laser output, it has a significant effect in many sensitive applications including test and evaluation of optical passive components and fibers and Dense Wavelength Division Multiplexing, Wavelength Division Multiplexing and optical fiber data-transmission systems. Consequently, there is a need to filter out SSE and ASE background radiation from the output of grating-tuned, external cavity laser systems to obtain a coherent, narrow-linewidth, noise-free output laser beam.

A few types of grating-tuned external cavity laser systems that could suppress SSE and ASE background noise have been described in the publications, “Using Diode Lasers for Atomic Physics”, by Carl E. Wieman and Leo Hollberg, Review of Scientific Instruments, vol. 62, pages 1-19, January, 1991 and “Impact of Source Spontaneous Emission (SSE) on the Measurement of DWDM Components”, by Edgar Leckel et al. These systems insert a beam coupler, usually consisting of an optical flat, into the grating-tuned external feedback cavity, along the laser beam path, between coupler partially reflects the laser beam out of the cavity.

FIG. 2 shows a schematic representation of a tunable laser source constructed by Hewlett-Packard Co. based on the concept described in the above-cited publications. Laser system <b>200</b> consists of diffraction grating <b>210</b>, waveguiding device <b>232</b>, laser diode <b>250</b>, tuning reflector <b>260</b>, beam splitter <b>292</b>, reflection mirror <b>294</b> and optical lens <b>296</b>.

Laser diode <b>250</b> is disposed to generate a laser beam which is incident at a grazing angle upon diffraction grating <b>210</b>. Beam splitter <b>292</b> is located along an optical path between laser diode <b>250</b> and diffraction grating <b>210</b> such that it intercepts a feedback light radiation component diffracted by diffraction grating <b>210</b>. Reflection mirror <b>294</b> is disposed to intercept a light radiation component diverted by beam splitter <b>292</b>. Optical lens <b>296</b> is disposed along an optical path between reflection mirror <b>294</b> and waveguiding device <b>232</b>.

In operation, laser diode <b>250</b> generates a laser beam which is incident on diffraction grating <b>210</b> at a grazing angle. Part of this beam is reflected by diffraction grating <b>210</b> to provide a conventional laser output (not shown in FIG. <b>2</b>). Diffraction grating <b>210</b> also diffracts a feedback light radiation component, which propagates back into laser diode <b>250</b> from the retroreflection of tuning reflector <b>260</b>. Beam splitter <b>292</b> intercepts and partially reflects the feedback light radiation component, thereby giving rise to a diverted light radiation component. The diverted light radiation component consists of a laser beam, an angularly-separated SSE light component and an angularly-separated ASE light component. The diverted light radiation component reflects off reflection mirror <b>294</b> and is incident on optical lens <b>296</b>. Optical lens <b>296</b> refracts the incident diverted light radiation while maintaining the angular separation between its three constituent components. Upon refraction by optical lens <b>296</b>, the laser beam component of the diverted light radiation is coupled into waveguiding device <b>232</b> while the angularly-separated SSE and ASE components are filtered out, thereby giving rise to a low-noise laser beam (not shown in FIG. <b>2</b>).

The laser system described above and shown in the embodiment of FIG. 2 has a number of disadvantages. A disadvantages of the laser system of FIG. 2 is that both the conventional output laser beam and the low-noise laser beam coupled into waveguiding device <b>232</b> have reduced optical power due to optical power losses and additional optical dispersion which occur in the laser cavity due to the introduction of beam splitter <b>292</b>. A further disadvantage of this laser system is that the introduction of beam splitter <b>292</b> in the laser cavity modifies the cavity length, and consequently, component positions have to be carefully adjusted to achieve mode-free tuning for the output laser beams. Another disadvantage of the laser system shown in FIG. 2 is that introduction of beam splitter <b>292</b> into the laser cavity increases the lasing theshold of the laser cavity, therefore increasing the instability of the laser operation of laser diode <b>250</b>.

Considering the limitations associated with grating-tuned, external cavity laser systems in the prior art, including the disadvantages described above, there is a need for a grating-tuned, external cavity laser system which can produce a continuously-tunable laser output with suppressed SSE and ASE background noise over the entire laser tuning range and with automatic wavelength and power tracking capability.

SUMMARY OF THE INVENTION

In an aspect, the invention relates to an external cavity diode laser system comprising a dispersion unit; a gain element producing coherent light incident upon the dispersion unit, and the dispersion unit dispersing the incident coherent light into dispersed light, the dispersed light comprising a reflected diffraction beam and at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission; a guiding dispersion unit that guides the dispersed light diffracted upon it from the dispersion unit while maintaining an angular separation between the reflected diffraction beam and at least on of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission; and a physical filtering device that physically filters the reflected diffraction beam from the spatially separated at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission guided to the physical filtering device by the guiding unit to produce a low-noise laser beam.

In another aspect, the invention relates to a laser system comprising an external cavity diode laser that emits dispersed light, and the dispersed light comprising a reflected diffraction beam and at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission; a guiding dispersion unit, positioned along the beam path of the reflected diffraction beam; and a physical filtering device positioned along a beam path of the reflected diffracted beam that physically filters the reflected diffraction beam from the at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission to produce a low-noise laser beam.

In still another aspect, the invention relates to a method comprising providing an external cavity diode laser that emits a reflected diffraction beam and at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission; dispersing the reflected diffraction beam a first time along a propagation direction by disposing a dispersion unit in the optical path of the reflected diffraction optical beam; and physically filtering the reflected diffraction beam from the at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission.

In an aspect, the invention relates to an external cavity diode laser system comprising first dispersive means; means for producing coherent light incident upon the first dispersive means, the first dispersive means dispersing the incident coherent light into dispersed light, the dispersed light comprising a reflected diffraction beam and at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission; and physically-filtering means, disposed along a beam path of the reflected diffraction beam, for physically filtering the reflected diffraction beam from the at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission to produce a low-noise laser beam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art grating-tuned, external cavity laser system.

FIG. 2 shows another prior art grating-tuned, external cavity laser system including a beam splitter.

FIG. 3 shows a continuously-tunable, low-noise, grating-tuned, external cavity laser system according to the present invention with a guiding element comprising a flat reflection mirror and a beam collector comprising an optical lens.

FIG. 4 shows a representation of the light radiation pattern in the X-Y focal plane of the beam collector shown in FIG. 3 FIG. 5 shows a simulation of the effectiveness of SSE and ASE filtering achieved by an embodiment of the present invention.

FIG. 6 shows another embodiment of the present invention with a laser diode acting as a light source and a collimation lens acting as a light collimating device.

FIG. 7 shows another embodiment of the present invention with a concave mirror acting as a beam collector.

FIG. 8 shows another embodiment of the present invention with a laser diode acting as a light source, a collimation lens acting as a light collimating device and a concave mirror acting as beam collector.

FIG. 9 shows another embodiment of the present invention with a concave mirror acting as both a guiding element and a beam collector.

FIG. 10 shows another embodiment of the present invention with a laser diode acting as a light source, a collimation lens acting as a light collimating device and a concave mirror acting as both a guiding element and beam collector.

FIG. 11 shows another embodiment of the present invention with a dispersion unit acting as both a guiding element and beam collector.

FIG. 12 shows another embodiment of the present invention with a laser diode acting as a light source, a collimation lens acting as a light collimating device and a dispersion unit acting as both a guiding element and beam collector.

FIG. 13 shows another embodiment of the present invention with an optical transmission pinhole acting as a narrow, band-pass filter.

FIG. 14 shows another embodiment of the present invention with an alternative disposition of certain elements.

FIG. 15 shows a schematic representation of the embodiment of FIG. <b>14</b>.

FIG. 16 shows a representation of the light radiation pattern in the X-Y focal plane of the beam collector shown in FIG. <b>14</b>.

FIG. 17 shows a simulation of the effectiveness of SSE and ASE filtering achieved by an embodiment of the present invention.

FIG. 18 shows another embodiment of the present invention with a laser diode acting as a light source and a collimation lens acting as a light collimating device.

FIG. 19 shows another embodiment of the present invention with a concave mirror acting as a beam collector.

FIG. 20 shows another embodiment of the present invention with a laser diode acting as a light source, a collimation lens acting as a light collimating device and a concave mirror acting as beam collector.

FIG. 21 shows another embodiment of the present invention with a dispersion unit acting as both a guiding element and beam collector.

FIG. 22 shows another embodiment of the present invention with a laser diode acting as a light source, a collimation lens acting as a light collimating device and a dispersion unit acting as both a guiding element and beam collector.

FIG. 23 shows another embodiment of the present invention with an optical transmission pinhole acting as a narrow, band-pass filter.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, a grating-tuned, external cavity laser system and a method to suppress SSE and ASE background light noise is described. The system and method disclosed herein recycle and effectively employ optical power dissipated and wasted by prior art external cavity laser systems to produce a continuously-tunable, narrow-bandwidth laser beam with low SSE and ASE light noise in addition to the conventional laser beam associated with prior art external cavity laser systems.

The present invention has numerous advantages over the laser systems in the prior art. For example, an advantage of the current invention over the laser system shown in FIG. 2 is that since the present invention does not insert any optical devices into the laser cavity, the present invention avoids perturbing the operation of the master laser cavity operation in general. As a result, the grating-tuned external cavity can directly and fully utilize its mode-hop-free tuning capacity to tune the laser wavelength over large bandwidths without any further adjustments and repositioning of optical components. In contrast, the presence of an optical beam splitter into the laser cavity of the laser system shown in FIG. 2 modifies the length of the laser cavity and requires repositioning of different components, as discussed above. Additional advantages of the present invention over the prior art, including over the system of FIG. 2, will be discussed below in conjunction with different embodiments of the present invention, or will be apparent to one skilled in the art.

FIG. 3 shows a tuning arrangement for SSE and ASE suppression in a grating-tuned external cavity laser with dual laser beam output. Dual-beam laser system <b>300</b> comprises pivot <b>302</b>, base <b>304</b>, plane reflector <b>306</b>, gain medium <b>308</b>, dispersion unit <b>310</b>, tuning reflector <b>312</b>, rotatable unit <b>314</b>, output laser beam <b>316</b>, first-order diffracted radiation <b>318</b>, reflected diffraction beam <b>320</b>, guiding mirror <b>322</b>, collector incident light <b>324</b>, beam collector <b>326</b>, focused light spot <b>328</b>, optical coupling device <b>330</b>, waveguiding device <b>332</b>, low noise laser beam <b>334</b>, angularly-separated SSE <b>336</b>, angulary-separated ASE <b>338</b>, and coupling unit <b>390</b>.

A proximal end of rotatable unit <b>314</b> is pivotably connected to base <b>304</b> by pivot <b>302</b>. Tuning reflector <b>312</b> is mechanically coupled to rotatable unit <b>314</b> forming an acute angle with respect to dispersion unit <b>310</b>, which is mechanically coupled to an upper surface of base <b>304</b>. In a preferred embodiment, dispersion unit <b>310</b> comprises a diffraction grating and tuning reflector <b>312</b> comprises a Porro prism. Use of Porro prisms as reflectors is well-known in the art and is described in Eugene Hecht, <i>Optics</i>, Addison-Wesley Publishing Company, Inc. (1987), p. 168. In an alternative embodiment, tuning reflector <b>312</b> comprises a reflection mirror.

Beam collector <b>326</b> is mechanically coupled to rotatable unit <b>314</b> distally from tuning reflector <b>312</b> with respect to pivot <b>302</b>. Guiding mirror <b>322</b> is mechanically coupled to base <b>304</b> and is located in line-of-sight of beam collector <b>326</b>. Coupling unit <b>390</b> comprises guiding mirror <b>322</b>, beam collector <b>326</b> and optical coupling device <b>330</b>. Plane reflector <b>306</b> and gain medium <b>308</b> are mechanically coupled to base <b>304</b> and are disposed to produce a laser beam which is incident on dispersion unit <b>310</b> at a grazing angle, thereby generating output laser beam <b>316</b>, first-order diffracted radiation <b>318</b>, reflected diffraction beam <b>320</b>, angularly separated SSE <b>336</b> and angularly-separated ASE <b>338</b>. In a preferred embodiment, plane reflector <b>306</b> comprises a rear facet of a laser diode. Beam collector <b>326</b> is disposed along an optical path between guiding mirror <b>322</b> and optical coupling device <b>330</b> of waveguiding device <b>332</b>. In a preferred embodiment, optical coupling device <b>330</b> comprises an optical fiber aperture or the tip of a fiberoptic cable, and waveguiding device <b>332</b> comprises a single-mode or a multi-mode fiberoptic cable.

In operation, rotating arm <b>314</b> pivots around pivot <b>302</b> such that tuning reflector <b>312</b> and beam collector <b>326</b> move relative to dispersion unit <b>310</b> and guiding mirror <b>322</b>. Plane reflector <b>306</b> and gain element <b>308</b> generate coherent light radiation comprising a laser beam which is incident on dispersion unit <b>310</b> at a grazing angle. Part of this laser beam is reflected as output laser beam <b>316</b>. Output laser beam <b>316</b> exits dual-beam laser system <b>300</b> and represents a conventional laser beam generally associated in the art with grating-tuned external cavity lasers. The rest of the laser beam incident on dispersion unit <b>310</b> is diffracted and reflected to generate a light radiation pattern which includes first-order diffracted radiation <b>318</b>, reflected diffraction beam <b>320</b>, angularly-separated SSE <b>336</b> and angulary-separated ASE <b>338</b>. First-order diffracted radiation <b>318</b> retro-reflects off tuning reflector <b>312</b> and is again incident on dispersion unit <b>310</b>. Upon further diffraction and reflection by dispersion unit <b>310</b>, a portion of first-order diffracted radiation <b>318</b> enters gain element <b>308</b> and reflects off plane reflector <b>306</b> thereby forming an external feedback laser cavity for the dual-beam laser system <b>300</b>.

Reflected diffraction beam <b>320</b> comprises a laser beam with a wavelength equal to the wavelength of output laser beam <b>316</b>. Angularly-separated SSE <b>336</b> and angulary-separated ASE <b>338</b> comprise incoherent light radiation which spans a broad range of wavelengths and which propagates away from dispersion unit <b>310</b> on optical paths which form acute angles with the direction of propagation of reflected diffraction beam <b>320</b>. Alternatively stated, angularly-separated SSE <b>336</b> and angulary-separated ASE <b>338</b> diverge from reflected diffraction beam <b>320</b> as they propagate away from dispersion unit <b>310</b>.

Reflected diffraction beam <b>320</b>, angularly-separated SSE <b>336</b> and angulary-separated ASE <b>338</b> propagate away from dispersion unit <b>310</b> on diverging optical paths and reflect off guiding mirror <b>322</b> to generate collector incident light <b>324</b>. Beam collector <b>326</b> refracts collector incident light <b>324</b> and concentrates it into a number of discrete light spots including focused light spot <b>328</b>. A spatial propagation separation of reflected diffraction beam <b>320</b> with respect to and angularly-separated SSE <b>336</b> and angulary-separated ASE <b>338</b> is maintained by coupling unit <b>390</b> upon reflection by guiding mirror <b>322</b> and refraction by beam collector <b>326</b>, and is transposed into a spatial or angular separation of the discrete light spots formed by beam collector <b>326</b>.

Each light spot comprises light with a narrow range of wavelengths. Focused light spot <b>328</b> comprises light from reflected diffraction beam <b>320</b>, which has a narrow wavelength band centered on the wavelength of output laser beam <b>316</b>. The total light energy contained in focused light spot <b>328</b> is significantly higher than the total light energy of any of the other light spots. Focused light spot <b>328</b> is coupled into optical coupling device <b>330</b> and propagates through waveguiding device <b>332</b>, thereby generating low noise laser beam <b>334</b>. The other focused light spots comprise light from angularly-separated SSE <b>336</b> and angularly-separated ASE <b>338</b>. These focused light spots are physically filtered out by coupling unit <b>390</b> by not being coupled into waveguiding device <b>332</b>, and therefore the SSE and ASE light noise is suppressed from low noise laser beam <b>334</b>.

The propagation angles with respect to base <b>304</b> and the wavelengths of both output laser beam <b>316</b> and reflected diffraction beam <b>320</b> depend on the angle formed by dispersion unit <b>310</b> with the reflecting surface of tuning reflector <b>312</b>, which may be adjusted by pivoting rotatable unit <b>314</b> around pivot <b>302</b>. The propagation angles of reflected diffraction beam <b>320</b> with respect to base <b>304</b> and guiding mirror <b>322</b> determine the optical propagation pattern of collector incident light <b>324</b>. As previously discussed, collector incident light <b>324</b> is refracted by beam collector <b>326</b> to generate focused light spot <b>328</b>. Focused light spot <b>328</b> is located in the focal plane of beam collector <b>326</b> and its position depends on the optical characteristics of beam collector <b>326</b> and on the propagation pattern and wavelength structure of collector incident light <b>324</b>. Since reflected diffraction beam <b>320</b> which is comprised in collector incident light <b>324</b> has substantially the same wavelength as output laser beam <b>316</b>, and since the wavelength of output laser beam <b>316</b> may be tuned by pivoting rotatable unit <b>314</b> around pivot <b>302</b>, the spatial distribution of focused light spot <b>328</b> can be adjusted by pivoting rotatable unit <b>314</b>. The topography and elements of dual-beam laser system <b>300</b> are selected such that pivoting of rotatable unit <b>314</b> results both in controlled wavelength tuning of output laser beam <b>316</b> and in stable coupling of focused light spot <b>328</b> into optical coupling device <b>330</b>.

The following discussion provides a mathematical description for the structure and operation of the embodiment shown in FIG. <b>3</b>. Despite the specific nature of the following discussion, it may be applied generally in principle to other embodiments of the present invention. As discussed above, reflected diffraction beam <b>320</b> comprises a desired coherent light radiation component (i.e., a laser beam with a wavelength substantially identical with the wavelength of output laser beam <b>316</b>). In contrast, angularly-separated SSE <b>336</b> and angularly-separated ASE <b>338</b> comprise undesired incoherent noise background light radiation which generally covers the full emission band of gain medium <b>308</b> and couple with reflected diffraction beam <b>320</b> in space and time. To suppress angularly-separated SSE <b>336</b> and angularly-separated ASE <b>338</b>, spatial (i.e. angular) separation of these components and spatial narrow band-pass filtering are required. In the present invention, spatial (i.e. angular) separation is provided by the angular and spectral dispersion introduced by dispersion unit <b>310</b> and spatial narrow band-pass filtering is provided by coupling unit <b>390</b> through adequate placement of optical coupling device <b>330</b> relative to the location of focused beam spots <b>328</b>.

Upon incidence on dispersion unit <b>310</b>, the laser beam generated by gain medium <b>308</b> and plane reflector <b>306</b> is dispersed into a radiation pattern which includes output laser beam <b>316</b>, first-order diffracted radiation <b>318</b> and reflected diffraction beam <b>320</b>. Light comprising these three components propagates along different wavelength-dependent paths, forming angles θ(λ) with respect to dispersion unit <b>310</b>. If the laser beam generated by gain medium <b>308</b> and plane reflector <b>306</b> forms an angle of incidence θ₀with respect to the dispersion unit <b>310</b> and if the spatial period of dispersion unit <b>310</b> is denoted by d, the angle θ(λ) can be expressed as, <maths><math><mrow><mrow><mi>θ</mi><mo></mo><mrow><mo>(</mo><mi>λ</mi><mo>)</mo></mrow></mrow><mo>=</mo><mrow><mrow><mi>arcsin</mi><mo></mo><mrow><mo>[</mo><mrow><mfrac><mi>λ</mi><mi>d</mi></mfrac><mo>-</mo><mrow><mi>sin</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><msub><mi>θ</mi><mn>0</mn></msub></mrow></mrow><mo>]</mo></mrow></mrow><mo>.</mo></mrow></mrow></math><img id="EMI-M00001" file="US06606340-20030812-M00001.TIF" img-content="math" img-format="tif" alt="embedded image" /><attachments><attachment idref="MATHEMATICA-00001" attachment-type="nb" file="US06606340-20030812-M00001.NB" /></attachments></maths>

The intensity of collector incident light <b>324</b> in the X-Y focal plane of beam collector <b>326</b> is described by a two-dimensional (x, y) equation which includes an angular-cone distribution-function representing the beam focusing effect of beam collector <b>326</b>,

<maths><formula-text><i>I</i>(λ,<i>x,y</i>,Ω)=<i>I</i>(λ)ζ(<i>x−x</i>_λ<i>,y−y</i>_λ,ƒ,Ω),</formula-text></maths>

where a normalized arbitrary distribution function ζ(x−x_λ, y−y_λ,ƒ,Ω)

provides a light intensity distribution for focused light spot <b>328</b> with beam center (x_λ, y_λ, f) in the X-Y focal plane of beam collector <b>326</b> as illustrated in FIG. <b>4</b>.

If the grating of diffraction grid <b>310</b> exhibits a one-dimensional variation, coordinates x_λ, and y_λcan be expressed as x_λ=ƒ tan θ(λ) and y_λ=0, where f represents the focal length of the beam collector <b>326</b>. In a preferred embodiment, beam collector <b>326</b> comprises a convex lens and f represents the focal length of the convex lens. In the X-Y focal plane of beam collector <b>326</b>, optical coupling device <b>330</b> is aligned with the center of focused light spot <b>328</b> to receive the light with laser wavelength λ_L. In a preferred embodiment, optical coupling device <b>330</b> comprises a single-mode fiber, a multi-mode transparent fiber or a waveguide designed for coupling a light beam. Through proper alignment with focused light spot <b>328</b>, optical coupling device <b>330</b> receives only light with wavelength λ_Lby filtering out light with other wavelengths than λ_L.

The light energy E(λ) coupled into waveguiding device <b>332</b> by optical coupling device <b>330</b> is determined by convolution of the X-Y focal light intensity distribution function I(λ,x,y,Ω) with both an optical aperture function ρ(x−x_λ_{<sub2>L</sub2>},y−y_λ_{<sub2>L</sub2>},z−ƒ,Ω) characteristic to optical coupling device <b>330</b> and with a numerical aperture function κ(λ) representing the average insertion or surface-reflection light coupling loss over the entire area of optical coupling device <b>330</b>,

<maths><formula-text><i>E</i>^(fiber)(λ)=<i>I</i>(λ)(1−κ(λ))∫∫∫ρ(<i>x−x</i>_λ_{<sub2>L</sub2>}<i>,y−y</i>_λ_{<sub2>L</sub2>},ƒ,Ω)·ζ(<i>x−x</i>_λ<i>,y−y</i>_λ,ƒ,Ω)<i>dxdydΩ=E</i>(λ)(1−κ(λ))σ(λ)</formula-text></maths>

where the filtering effect of the beam coupling device is represented by a filtering function

<maths><formula-text>σ(λ)=∫∫∫ρ(<i>x−x</i>_λ_{<sub2>L</sub2>}<i>,y−y</i>_λ_{<sub2>L</sub2>},ƒ,Ω)·ζ(<i>x−x</i>_λ<i>,y−y</i>_λ,ƒ,Ω)<i>dxdydΩ</i></formula-text></maths>

If the numerical aperture of optical coupling device <b>330</b> is larger than the divergence of focused light spot <b>328</b> and if the area of optical coupling device <b>330</b> is larger than the size of focused light spot <b>328</b> at the laser wavelength λ=λ_L, all the light of the respective beam spot is coupled into waveguiding device <b>332</b>. The energy of the light coupled into the fiber can therefore be expressed as,

<maths><formula-text><i>E</i>^(fiber)(λ)≈<i>E</i>(λ_L)(1−κ(λ_L)).</formula-text></maths>

FIG. 4 shows a representation of the X-Y focal plane of beam collector <b>326</b> from FIG. <b>3</b> and illustrates how the present invention achieves spatial narrow band-pass filtering for light coupled into waveguiding device <b>332</b>. FIG. 4 shows the focal plane <b>400</b> of beam collector <b>326</b>. Focal plane <b>400</b> includes an aligned light spot <b>402</b>, a misaligned light spot <b>404</b> and an optical aperture <b>406</b>. Consistent with the previous discussion, beam collector <b>326</b> refracts collector incident light <b>324</b> and concentrates it into a number of discrete light spots in the focal plane <b>400</b> of beam collector <b>326</b>. The spatial position of each light spot depends upon the wavelength of the light associated with that particular light spot.

Aligned light spot <b>402</b> represents focused light spot <b>328</b> from FIG. 3 which comprises light of substantially wavelength λ_L. Aligned light spot <b>402</b> is centered at coordinates (x_λL, y_λL) and has a radius of r_λ_{<sub2>L</sub2>}. The radius r_λ_{<sub2>L </sub2>}is selected such that the resulting circular area includes only light with an intensity of at least 1/e of the peak value existing within aligned light spot <b>402</b>. Aligned light spot <b>402</b> is concentrically collocated with optical aperture <b>406</b>. If the radius r₀of optical aperture <b>406</b> is larger than the radius r_λ_{<sub2>L </sub2>}of aligned light spot <b>402</b>, aligned light spot <b>402</b> is fully contained within optical aperture <b>406</b> and light from aligned light spot <b>402</b> may be fully coupled into waveguiding device <b>332</b>. In contrast, misaligned light beam <b>404</b> (which includes SSE and ASE radiation) is centered at coordinates (x_λ, y_λ) and is not fully contained within optical aperture <b>406</b>. Consequently, light associated with misaligned light spot <b>404</b> cannot be fully coupled into waveguiding device <b>332</b> and is therefore at least partially filtered out.

As a result of spatial narrow band-pass filtering, therefore, for any light as well as SSE and ASE background noise radiation with wavelengths other than λ_L(λ≠λ_L), the energy coupled into waveguiding device <b>332</b> upon proper alignment of optical aperture <b>406</b> with aligned light spot <b>402</b> is minimized such that E^(fiber)(λ)≈0.

Referring to FIG. 4, for r_λ≦r₀, the filtering function associated with optical aperture <b>406</b> can be expressed as

<maths><formula-text>σ(λ)=π⁻¹<i>r</i>_λ⁻¹·∫∫∫ρ(<i>x−x</i>_λ_{<sub2>L</sub2>}<i>,y−y</i>_λ_{<sub2>L</sub2>},ƒ,Ω)·ζ(<i>x−x</i>_λ<i>,y−y</i>_λ,ƒ,Ω)<i>dxdydΩ.</i></formula-text></maths>

For r_λ≧r₀, however, the filtering function can be represented by

<maths><formula-text>σ(λ)=π⁻¹<i>r</i>_λ_{<sub2>L</sub2>}^−z·∫∫∫ρ(<i>x−x</i>_λ_{<sub2>L</sub2>}<i>,y−y</i>_λ_{<sub2>L</sub2>},ƒ,Ω)·ζ(<i>x−x</i>_λ<i>,y−y</i>_λ,ƒ,Ω)<i>dxdydΩ.</i></formula-text></maths>

The optical aperture function of optical aperture <b>406</b> and the normalized distribution function describing the light intensity distribution for aligned light spot <b>402</b> can then be approximated by,

<maths><formula-text>ρ(<i>x−x</i>_λ_{<sub2>L</sub2>}<i>,y−y</i>_λ_{<sub2>L</sub2>},ƒ,Ω)≡μ(<i>x−x</i>_λ_{<sub2>L</sub2>}<i>,y−y</i>_λ_{<sub2>L</sub2>},ƒ)Θ(Ω)Γ(<i>r</i>_λ_{<sub2>L</sub2>}<i>−r</i>),</formula-text></maths>

and respectively,

ζ(<i>x−x</i>_λ<i>,y−y</i>_λ,ƒ,Ω)≡τ(<i>x−x</i>_λ<i>,y−y</i>_λ,ƒ)Θ_FIBER(Ω)Γ(<i>r</i>_λ<i>−r</i>′),

where Ω_Lrepresents the spherical angle of the light intensity distribution of aligned light spot <b>402</b>, Ω_FIBERrepresents the numerical aperture of optical aperture <b>406</b>, and the following formulas apply:

<maths><formula-text><i>r</i>={square root over ((<i>x−x</i>_λ_{<sub2>L</sub2>})²+(<i>y−y</i>_λ_{<sub2>L</sub2>})²)};</formula-text></maths>

<maths><formula-text><i>r</i>′={square root over ((<i>x−x</i>_λ)²+(<i>y−y</i>_λ)²)};</formula-text></maths>

<maths><math><mrow><mrow><mrow><mi>Γ</mi><mo></mo><mrow><mo>(</mo><mrow><msub><mi>r</mi><msub><mi>λ</mi><mi>L</mi></msub></msub><mo>-</mo><mi>r</mi></mrow><mo>)</mo></mrow></mrow><mo>=</mo><mrow><mo>{</mo><mtable><mtr><mtd><mn>1</mn></mtd><mtd><mrow><mrow><msub><mi>r</mi><msub><mi>λ</mi><mi>L</mi></msub></msub><mo>-</mo><mi>r</mi></mrow><mo>≥</mo><mn>0</mn></mrow></mtd></mtr><mtr><mtd><mn>0</mn></mtd><mtd><mrow><mrow><msub><mi>r</mi><msub><mi>λ</mi><mi>L</mi></msub></msub><mo>-</mo><mi>r</mi></mrow><mo>&lt;</mo><mn>0</mn></mrow></mtd></mtr></mtable><mo>}</mo></mrow></mrow><mo>;</mo></mrow></math><math><mrow><mrow><mrow><mi>Γ</mi><mo></mo><mrow><mo>(</mo><mrow><msub><mi>r</mi><mi>λ</mi></msub><mo>-</mo><mi>r</mi></mrow><mo>)</mo></mrow></mrow><mo>=</mo><mrow><mo>{</mo><mtable><mtr><mtd><mn>1</mn></mtd><mtd><mrow><mrow><msub><mi>r</mi><mi>λ</mi></msub><mo>-</mo><mi>r</mi></mrow><mo>≥</mo><mn>0</mn></mrow></mtd></mtr><mtr><mtd><mn>0</mn></mtd><mtd><mrow><mrow><msub><mi>r</mi><mi>λ</mi></msub><mo>-</mo><mi>r</mi></mrow><mo>&lt;</mo><mn>0</mn></mrow></mtd></mtr></mtable><mo>}</mo></mrow></mrow><mo>;</mo></mrow></math><img id="EMI-M00002" file="US06606340-20030812-M00002.TIF" img-content="math" img-format="tif" alt="embedded image" /><attachments><attachment idref="MATHEMATICA-00002" attachment-type="nb" file="US06606340-20030812-M00002.NB" /></attachments></maths><maths><math><mrow><mrow><mi>Θ</mi><mo></mo><mrow><mo>(</mo><mi>Ω</mi><mo>)</mo></mrow></mrow><mo>≈</mo><mrow><mo>{</mo><mtable><mtr><mtd><mn>1</mn></mtd><mtd><mrow><mi>Ω</mi><mo>≤</mo><msub><mi>Ω</mi><mi>L</mi></msub></mrow></mtd></mtr><mtr><mtd><mn>0</mn></mtd><mtd><mrow><mi>Ω</mi><mo>&gt;</mo><msub><mi>Ω</mi><mi>L</mi></msub></mrow></mtd></mtr></mtable><mo>}</mo></mrow></mrow></math><img id="EMI-M00003" file="US06606340-20030812-M00003.TIF" img-content="math" img-format="tif" alt="embedded image" /><attachments><attachment idref="MATHEMATICA-00003" attachment-type="nb" file="US06606340-20030812-M00003.NB" /></attachments></maths>

(This formula represents the angular distribution function of aligned light spot <b>402</b>); <maths><math><mrow><mrow><msub><mi>Θ</mi><mi>FIBER</mi></msub><mo></mo><mrow><mo>(</mo><mi>Ω</mi><mo>)</mo></mrow></mrow><mo>≈</mo><mrow><mo>{</mo><mtable><mtr><mtd><mn>1</mn></mtd><mtd><mrow><mi>Ω</mi><mo>≤</mo><msub><mi>Ω</mi><mi>FIBER</mi></msub></mrow></mtd></mtr><mtr><mtd><mn>0</mn></mtd><mtd><mrow><mi>Ω</mi><mo>&gt;</mo><msub><mi>Ω</mi><mi>FIBER</mi></msub></mrow></mtd></mtr></mtable><mo>}</mo></mrow></mrow></math><img id="EMI-M00004" file="US06606340-20030812-M00004.TIF" img-content="math" img-format="tif" alt="embedded image" /><attachments><attachment idref="MATHEMATICA-00004" attachment-type="nb" file="US06606340-20030812-M00004.NB" /></attachments></maths>

(This formula represents the numerical function of optical aperture <b>406</b>);

<maths><formula-text><i>x</i>_λ=ƒ tan θ(λ);</formula-text></maths>

<i>y</i>_λ=0;

<maths><formula-text><i>x</i>_λ_{<sub2>L</sub2>}=ƒ tan θ(λ_L); and</formula-text></maths>

<maths><formula-text><i>y</i>_λ_{<sub2>L</sub2>}=0.</formula-text></maths>

(r_λ, r_λ_{<sub2>L </sub2>}and r₀have been previously defined).

FIG. 5 shows a simulation of the effectiveness of SSE and ASE filtering achieved by an embodiment of the present invention for r_λ_{<sub2>L </sub2>}=r₀0, ƒ=1000·r₀, d=1 μm, Ω_L≦Ω_FIBERand assuming a Gaussian light intensity distribution for aligned light spot <b>402</b>. Over the emission band of gain medium <b>308</b> of FIG. 3, the present invention filters out SSE and ASE background noise radiation at all wavelengths other than the desired laser wavelength λ_L, which is shown in FIG. 5 to be approximately 1.54 μm. Consequently, the only light coupled into waveguiding device <b>332</b> is light with the desired wavelength, λ_L.

The present invention provides numerous advantages over the prior art. For convenience, and to take advantage of the detailed description provided in connection with the embodiment shown in FIG. 3, a number of advantages of the present invention will be discussed here with particular reference to the embodiment of FIG. <b>3</b>. These advantages, however, may also apply to other embodiments of the present invention disclosed herein. Additionally, embodiments of the present invention may have additional advantages, some of which may be further described below.

An advantage of the embodiment of FIG. 3 is that it provides a means for the low noise laser beam <b>334</b> to track the laser wavelength of output laser beam <b>316</b> with automatic power coupling control as the wavelength of output laser beam <b>316</b> is continuously tuned through a broad range of wavelengths. Alternatively stated, the embodiment of FIG. 3 can maintain a maximum and constant level of light coupled into waveguiding device <b>332</b> while the wavelength of output laser beam <b>316</b>, and implicitly of low noise laser beam <b>334</b>, is tuned across a wide range of wavelengths. In the embodiment shown in FIG. 3, this advantage is achieved by appropriate selection of the physical dimensions of dual-beam laser system <b>300</b>.

As shown in FIG. 3, dispersion unit <b>310</b> and guiding mirror <b>322</b> are mechanically coupled to base <b>304</b> such that their normals form an angle φ₀. Beam collector <b>326</b> is mechanically coupled to rotatable unit <b>314</b> such that its focal axis forms an angle Φ₀with the normal of tuning reflector <b>312</b>. As a result, upon reflection off guiding mirror <b>322</b>, reflected diffraction beam <b>324</b> forms an angle α(λ_L) with the focal axis of beam collector <b>326</b>, where

<maths><formula-text>α(λ_L)=180°−2φ₀±Φ₀.</formula-text></maths>

The angle α(λ_L) is maintained constant as rotatable unit <b>314</b> pivots around pivot <b>302</b> to tune wavelength λ_Lthrough a broad range of wavelengths. Consequently, focused light spot <b>328</b> can be continuously coupled into waveguiding device <b>332</b> while wavelength λ_Lis tuned.

The automatic wavelength and power tracking features of the present invention could also be achieved through an active tracking system which would move optical coupling device <b>330</b> in response to positional variations of focused light spot <b>328</b> due to wavelength tuning in the laser system.

Such as system might employ a computer system coupled with a light sensor located in the proximity of optical aperture <b>406</b>. The light sensor would provide feedback data to the computer system to permit dynamic relocation of optical coupling device <b>330</b> in response to movement of focused light spot <b>328</b> to maintain stable light coupling into waveguiding device <b>332</b>.

Such a system would be difficult and expensive to implement, however, considering that the optical sensor would have to be inserted into the laser system and located in the proximity of optical aperture <b>406</b>. Further, optical aperture <b>406</b> would have to be independently mobile with respect to rotatable unit <b>314</b>, thereby requiring a complex mechanical coupling device with full two-dimensional movement capability. Such a coupling device would be difficult to implement considering the high degree of precision required for proper optical alignment of optical aperture <b>406</b> with focused light spot <b>328</b>. The inclusion of a complex mobile mechanical coupling device for optical aperture <b>406</b> would also significantly complicate the design and functionality of rotatable unit <b>314</b>, whose pivoting around pivot <b>302</b> must be accurately controlled but is highly sensitive to the mass and moment of the components coupled to rotatable unit <b>314</b>. In contrast, the present invention provides a system which automatically tracks and fully couples focused light spot <b>328</b> into optical aperture <b>406</b> without any active tracking components, therefore circumventing the limitations associated with an active tracking system.

Another advantage of the embodiment of FIG. 3 is that it may suppress SSE and ASE noise in a laser output of a grating-tuned, external cavity laser system. Referring to FIG. 3, the SSE and ASE noise present in the laser beam generated by gain medium <b>308</b> and plane reflector <b>306</b> is dispersed upon its incidence on dispersion unit <b>310</b>. Since most of the SSE and ASE noise consists of light with wavelengths that are different from the desired laser wavelength and cover the entire emission band of gain medium <b>308</b>, the grating dispersion redirects angularly-separated SSE <b>336</b> and angularly-separated ASE <b>338</b> in propagation directions divergent from the propagation path of reflected diffraction beam <b>320</b>. Coupling unit <b>390</b>, which comprises guiding mirror <b>322</b>, beam collector <b>326</b> and optical coupling device <b>330</b>, translates the angular separation of these beam propagation directions into a spatial distribution of light beam energy, which is coupled into waveguiding device <b>332</b> to generate low noise laser beam <b>334</b>. Consequently, the embodiment of FIG. 3 produces low noise laser beam <b>334</b> which is essentially free of SSE and ASE background noise.

Yet another advantage of the embodiment of FIG. 3 is that it provides an additional laser beam output for grating-tuned, external cavity laser systems by recovering optical energy traditionally wasted by grating-tuned, external cavity laser systems in the prior art and efficiently employing it in a novel approach to generate a non-conventional laser beam with an extremely low level of SSE and ASE noise. This additional laser beam is tuned at the same laser wavelength as the conventional laser beam, but exits the laser system through a separate output port.

A further advantage of the embodiment of FIG. 3 is that it provides a significant number of benefits without interfering with the functionality and classic design of conventional grating-tuned, external cavity laser systems. More specifically, since the embodiment of FIG. 3 generates low noise laser beam <b>334</b> by recycling previously-wasted optical energy, the power and general characteristics of output laser beam <b>316</b> are generally not affected. Additionally, the embodiment of FIG. 3 deviates from the classic design of conventional grating-tuned, external cavity laser systems only minimally, therefore decreasing the cost and uncertainties associated with radical design alterations.

FIG. 6 shows an alternative embodiment of the present invention. Dual-beam laser system <b>600</b> comprises pivot <b>602</b>, base <b>604</b>, dispersion unit <b>610</b>, tuning reflector <b>612</b>, rotatable unit <b>614</b>, output laser beam <b>616</b>, first-order diffracted radiation <b>618</b>, reflected diffraction beam <b>620</b>, guiding mirror <b>622</b>, collector incident light <b>624</b>, beam collector <b>626</b>, focused light spot <b>628</b>, optical coupling device <b>630</b>, waveguiding device <b>632</b>, low noise laser beam <b>634</b>, angularly-separated SSE <b>636</b>, angulary-separated ASE <b>638</b>, laser diode <b>650</b> and collimation lens <b>652</b>.

The structure of the embodiment shown in FIG. 6 is substantially identical with the structure of the embodiment shown in FIG. 3 except that the embodiment of FIG. 6 employs a laser diode <b>650</b> to replace the combination of the plane reflector <b>306</b> and the gain medium <b>308</b> from FIG. <b>3</b> and introduces a collimation lens <b>652</b> disposed along an optical path between laser diode <b>650</b> and dispersion unit <b>610</b>. Since the output of laser diode <b>650</b> generally exhibits an undesirable elliptically-divergent shape, collimation lens <b>652</b> is employed to collimate the light incident on dispersion unit <b>610</b> at a grazing angle. In a preferred embodiment, both facets of collimation lens <b>652</b> are treated with an anti-reflection coating to reduce light feedback from internal reflection within collimation lens <b>652</b>. Further, a facet of laser diode <b>650</b> oriented towards dispersion unit <b>610</b> is also treated with an anti-reflection coating to maximize power output of laser diode <b>650</b>.

In operation, the embodiment of FIG. 6 functions substantially the same as the embodiment of FIG. 3 because the light beam generated by laser diode <b>650</b> in conjunction with collimation lens <b>652</b> is substantially identical with the light beam produced by gain element <b>308</b> and plane reflector <b>306</b>. Consequently, the light incident at a grazing angle on diffraction grid <b>610</b> is substantially identical with the light incident at a grazing angle on diffraction grid <b>310</b>, and therefore the description provided for the embodiment of FIG. 3 generally applies to the embodiment of FIG. <b>6</b>.

FIG. 7 shows another alternative embodiment of the present invention. Dual-beam laser system <b>700</b> comprises pivot <b>702</b>, base <b>704</b>, plane reflector <b>706</b>, gain medium <b>708</b>, dispersion unit <b>710</b>, tuning reflector <b>712</b>, rotatable unit <b>714</b>, output laser beam <b>716</b>, first-order diffracted radiation <b>718</b>, reflected diffraction beam <b>720</b>, guiding mirror <b>722</b>, mirror incident light <b>724</b>, focused light spot <b>728</b>, optical coupling device <b>730</b>, waveguiding device <b>732</b>, low noise laser beam <b>734</b> angularly-separated SSE <b>736</b>, angulary-separated ASE <b>738</b>, and concave mirror <b>754</b>.

The structure of the embodiment of FIG. 7 is substantially identical with the structure of the embodiment of FIG. 3, except that the embodiment of FIG. 7 substitutes a concave mirror <b>754</b> for beam collector <b>326</b>. Concave mirror <b>754</b> is mechanically coupled to rotatable unit <b>714</b> such that a concave reflective surface of concave mirror <b>754</b> is oriented in the general direction of guiding mirror <b>722</b> to intercept mirror incident light <b>724</b>. Optical coupling device <b>730</b> and waveguiding device <b>732</b> are mechanically coupled to rotatable unit <b>714</b> on the same side of concave mirror <b>754</b> as guiding mirror <b>722</b>.

In operation, the embodiment shown in FIG. 7 functions substantially the same as the embodiment of FIG. <b>3</b>. Mirror incident light <b>724</b> is substantially identical with collector incident light <b>324</b> from FIG. <b>3</b>. Unlike in the embodiment of FIG. 3, however, mirror incident light <b>724</b> is not refracted by beam collector <b>326</b>, which comprises a lens, but is instead reflected by concave mirror <b>754</b>.

Concave mirror <b>754</b> is designed to reflect and focus mirror incident light <b>724</b> in a pattern substantially identical with the pattern experienced by the light refracted by beam collector <b>326</b> in the embodiment of FIG. <b>3</b>. Consequently, concave mirror <b>754</b> reflects and concentrates mirror incident light <b>724</b> into focused light spot <b>728</b> which is substantially identical to focused light spot <b>328</b> of FIG. <b>3</b>. The optical axis of concave reflection mirror <b>754</b> forms an angle ψ₀with the normal of tuning reflector <b>712</b>. To take advantage of the spatial filtering technique previously discussed in connection with the embodiment of FIG. 3, optical coupling device <b>730</b> is mechanically coupled to rotatable unit <b>714</b> such that focused light spot <b>728</b> is coupled into optical coupling device <b>330</b>.

Upon reflection by concave mirror <b>754</b>, reflected diffraction beam <b>720</b> propagates at an angle β(λ_L) with respect to the focal axis of concave mirror <b>754</b>. Referring to FIG. 7, angle β(λ_L) can be expressed as,

<maths><formula-text>β(λ_L)=180°−2φ₀±ψ₀.</formula-text></maths>

As indicated by this formula, angle β(λ_L) does not exhibit any dependence on wavelength or on the position of rotatable unit <b>714</b>, but is instead fully determined by initial selection and alignment of the components of dual-beam laser system <b>700</b>. To ensure full coupling of focused light spot <b>728</b> into optical coupling device <b>730</b>, the area of optical coupling device <b>730</b> must be larger than the effective size of focused light spot <b>728</b> and the numerical aperture of optical coupling device <b>730</b> must be larger than the convergence of focused light spot <b>728</b>. If these conditions are satisfied, proper initial design of dual-beam laser system <b>700</b> results in continuous and stable coupling of selected focused light spot <b>728</b> into waveguiding device <b>732</b> with simultaneous and effective filtering of SSE and ASE background light in the presence of laser tuning.

FIG. 8 shows yet another embodiment of the present invention. Dual-beam laser system <b>800</b> comprises pivot <b>802</b>, base <b>804</b>, dispersion unit <b>810</b>, tuning reflector <b>812</b>, rotatable unit <b>814</b>, output laser beam <b>816</b>, first-order diffracted radiation <b>818</b>, reflected diffraction beam <b>820</b>, guiding mirror <b>822</b>, collector incident light <b>824</b>, focused light spot <b>828</b>, optical coupling device <b>830</b>, waveguiding device <b>832</b>, low noise laser beam <b>834</b>, angularly-separated SSE <b>836</b>, angulary-separated ASE <b>838</b>, laser diode <b>850</b>, collimation lens <b>852</b> and concave mirror <b>854</b>.

The structure of the embodiment shown in FIG. 8 is substantially identical with the embodiment shown in FIG. 3, except that the embodiment of FIG. 8 includes the modifications introduced by the embodiments shown in FIG. <b>6</b> and FIG. <b>7</b>. More specifically, the embodiment of FIG. 8 employs a laser diode <b>850</b> to replace the combination of the plane reflector <b>306</b> and the gain medium <b>308</b> from FIG. <b>3</b> and introduces a collimation lens <b>852</b> disposed along an optical path between laser diode <b>850</b> and dispersion unit <b>810</b>, as discussed in connection with FIG. <b>6</b>. Additionally, the embodiment of FIG. 8 substitutes a concave mirror <b>854</b> for beam collector <b>326</b>, as described in conjunction with FIG. <b>7</b>. In a preferred embodiment, both facets of collimation lens <b>852</b> are treated with an anti-reflection coating to reduce light feedback from internal reflection within collimation lens <b>852</b>. Further, a facet of laser diode <b>850</b> oriented towards dispersion unit <b>810</b> is also treated with an anti-reflection coating to maximize power output of laser diode <b>850</b>.

In operation, both modifications operated to the embodiment shown in FIG. 8 perform substantially identical functions as the original elements they replace, as discussed in connection with the embodiments of FIG. 6 and FIG. <b>7</b>. Consequently, the descriptions provided for the embodiments shown in FIGS. 3, <b>6</b> and <b>7</b> also apply to the embodiment of FIG. <b>8</b>.

FIG. 9 shows yet another embodiment of the present invention. Dual-beam laser system <b>900</b> comprises pivot <b>902</b>, base <b>904</b>, plane reflector <b>906</b>, gain medium <b>908</b>, dispersion unit <b>910</b>, tuning reflector <b>912</b>, rotatable unit <b>914</b>, output laser beam <b>916</b>, first-order diffracted radiation <b>918</b>, reflected diffraction beam <b>920</b>, mirror incident light <b>924</b>, focused light spot <b>928</b>, optical coupling device <b>930</b>, waveguiding device <b>932</b>, low noise laser beam <b>934</b>, angularly-separated SSE <b>936</b>, angulary-separated ASE <b>938</b>, and concave guiding mirror <b>956</b>.

The structure of the embodiment shown in FIG. 9 is substantially identical with the structure of the embodiment shown in FIG. 3, except that the embodiment of FIG. 9 employs a concave guiding mirror <b>956</b> to replace both guiding mirror <b>322</b> and beam collector <b>326</b> of FIG. <b>3</b>. Concave guiding mirror <b>956</b> is mechanically coupled to base <b>904</b> such that its concave reflecting surface is directed in the general direction of rotatable unit <b>914</b> and its optical axis forms an angle φ₀with respect to the normal of dispersion unit <b>910</b>.

In operation, the embodiment shown in FIG. 9 functions substantially the same as the embodiment of FIG. <b>3</b>. Concave mirror <b>956</b> is designed to reflect and focus reflected diffraction beam <b>920</b> in a pattern substantially identical with the pattern exhibited by collector incident light <b>324</b> upon its refraction by beam collector <b>326</b> in the embodiment of FIG. <b>3</b>. Essentially, concave guiding mirror <b>956</b> is designed to operationally substitute both guiding mirror <b>322</b> and beam collector <b>326</b> of FIG. <b>3</b>. Consequently, concave guiding mirror <b>956</b> reflects and concentrates reflected diffraction beam <b>920</b> into focused light spot <b>928</b> which is substantially identical with focused light spot <b>328</b> from FIG. <b>3</b>. Analogously with the arrangement of FIG. 3, optical coupling device <b>930</b> is mechanically coupled to rotatable unit <b>914</b> such that it is oriented in the direction of concave guiding mirror <b>956</b> and is aligned to permit coupling of focused light spot <b>928</b> into optical coupling device <b>930</b>.

Upon reflection by concave guiding mirror <b>956</b>, reflected diffraction beam <b>924</b> forms an angle γ(λ_L) with the normal of tuning reflector <b>912</b>, where

<maths><formula-text>⊖(λ_L)=180°−2φ₀.</formula-text></maths>

As indicated by this formula, angle γ(λ_L) does not exhibit any dependence on wavelength or on the position of rotatable unit <b>914</b>, but is instead fully determined by initial selection and alignment of the components of dual-beam laser system <b>900</b>. Proper initial design of dual-beam laser system <b>900</b> results in continuous coupling of focused light spot <b>928</b> into optical coupling device <b>930</b> with simultaneous and effective filtering of SSE and ASE background light regardless of wavelength variations in the system as a result of laser tuning.

The particular arrangement of the embodiment of FIG. 9 results in an advantage. Specifically, as rotatable unit <b>914</b> pivots around pivot <b>902</b>, the distance between concave guiding mirror <b>956</b> and optical coupling device <b>930</b> varies because optical coupling device <b>930</b> is attached to, and moves together with, rotatable unit <b>914</b>. Consequently, since the focal length of concave guiding mirror <b>956</b> is fixed, and since optical coupling device <b>930</b> is initially located in the focal plane of concave mirror <b>956</b>, pivoting of rotatable unit <b>914</b> removes optical coupling device <b>930</b> from the focal plane of concave guiding mirror <b>956</b>. As a result, due to the inherent divergence of the light reflected by concave guiding mirror <b>956</b>, the size of focused light spot <b>328</b> will increase as it projects upon the optical coupling device <b>930</b> out of focus. This general concept may be applied to other embodiments of the present invention.

To maximize the amount of optical power coupled into waveguiding device <b>932</b>, the area of optical coupling device <b>930</b> must be larger than the effective size of focused light spot <b>328</b>, and the numerical aperture of optical coupling device <b>930</b> must be larger than the convergence of focused light spot <b>328</b>. Depending on the amplitude of movement of rotatable unit <b>914</b>, however, the size of focused light spot <b>928</b> could potentially exceed the effective size of optical coupling device <b>930</b>, therefore resulting in reduced coupling efficiency. This apparent inconvenience can be remedied by simultaneously moving optical coupling device <b>930</b> along the optical axis of concave guiding mirror <b>956</b> to compensate for any focal plane translation induced by pivoting of rotatable unit <b>956</b>.

FIG. 10 shows an alternative embodiment of the present invention. Dual-beam laser system <b>1000</b> comprises pivot <b>1002</b>, base <b>1004</b>, dispersion unit <b>1010</b>, tuning reflector <b>1012</b>, rotatable unit <b>1014</b>, output laser beam <b>1016</b>, first-order diffracted radiation <b>1018</b>, reflected diffraction beam <b>1020</b>, mirror incident light <b>1024</b>, focused light spot <b>1028</b>, optical coupling device <b>1030</b>, waveguiding device <b>1032</b>, low noise laser beam <b>1034</b>, angularly-separated SSE <b>1036</b>, angulary-separated ASE <b>1038</b>, laser diode <b>1050</b>, collimation lens <b>1052</b> and concave guiding mirror <b>1056</b>.

The structure of the embodiment shown in FIG. 10 is substantially identical with the structure of the embodiment shown in FIG. 9 except that the embodiment of FIG. 10 employs a laser diode <b>1050</b> to replace the combination of plane reflector <b>906</b> and gain medium <b>908</b> from FIG. <b>9</b> and introduces a collimation lens <b>1052</b> disposed along an optical path between laser diode <b>1050</b> and dispersion unit <b>1010</b>. Since the output of laser diode <b>1050</b> generally exhibits an undesirable elliptically-divergent shape, collimation lens <b>1052</b> is employed to collimate the light incident on dispersion unit <b>1010</b> at a grazing angle. In a preferred embodiment, both facets of collimation lens <b>1052</b> are treated with an anti-reflection coating to reduce light feedback from internal reflection within collimation lens <b>1052</b>. Further, a facet of laser diode <b>1050</b> oriented towards dispersion unit <b>1010</b> is also treated with an anti-reflection coating to maximize power output of laser diode <b>1050</b>.

In operation, the embodiment of FIG. 10 functions substantially the same as the embodiment of FIG. 9 because the light beam generated by laser diode <b>1050</b> in conjunction with collimation lens <b>1052</b> is substantially identical with the light beam produced by gain element <b>908</b> and plane reflector <b>906</b>. Consequently, the light incident at a grazing angle on diffraction grid <b>1010</b> is substantially identical with the light incident at a grazing angle on diffraction grid <b>910</b>, and therefore the description provided for the embodiment of FIG. 9 also applies to the embodiment of FIG. <b>10</b>.

FIG. 11 shows yet another alternative embodiment of the present invention. Dual-beam laser system <b>1100</b> comprises pivot <b>1102</b>, base <b>1104</b>, plane reflector <b>1106</b>, gain medium <b>1108</b>, dispersion unit <b>1110</b>, tuning reflector <b>1112</b>, rotatable unit <b>1114</b>, output laser beam <b>1116</b>, first-order diffracted radiation <b>1118</b>, reflected diffraction beam <b>1120</b>, beam collector incident light <b>1124</b>, beam collector <b>1126</b>, focused light spot <b>1128</b>, optical coupling device <b>1130</b>, waveguiding device <b>1132</b>, low noise laser beam <b>1134</b>, angularly-separated SSE <b>1136</b>, angulary-separated ASE <b>1138</b>, and guiding dispersion unit <b>1160</b>.

The structure of the embodiment of FIG. 11 is substantially identical with the structure of the embodiment of FIG. 3, except that in the embodiment of FIG. 11 a guiding dispersion unit <b>1160</b> substitutes guiding mirror <b>322</b> of FIG. <b>3</b> and beam collector <b>1126</b>, optical coupling device <b>1130</b> and waveguiding device <b>1132</b> are removed from rotatable unit <b>1114</b>. Guiding dispersion unit <b>1160</b> is mounted to the base <b>1104</b>, and substantially parallel with, dispersion unit <b>1110</b>, and is oriented towards dispersion unit <b>1110</b> to intercept reflected diffraction beam <b>1120</b>. Beam collector <b>1126</b>, optical coupling device <b>1130</b> and waveguiding device <b>1132</b> are mounted above dispersion unit <b>1110</b> and are fixed with respect to guiding dispersion unit <b>1160</b>.

In operation, the embodiment shown in FIG. 11 functions substantially the same as the embodiment of FIG. <b>3</b>. Reflected diffraction beam <b>1120</b> is substantially identical with reflected diffraction beam <b>320</b> from FIG. <b>3</b>. Unlike in the embodiment of FIG. 3, however, reflected diffraction beam <b>1120</b> is not reflected by guiding mirror <b>322</b>, but is instead diffracted by guiding dispersion unit <b>1160</b>.

Guiding dispersion unit <b>1160</b> is designed to diffract reflected diffraction beam <b>1120</b> in a pattern substantially identical with the pattern experienced by the light reflected by guiding mirror <b>322</b> in the embodiment of FIG. <b>3</b>. Consequently, beam collector incident light <b>1124</b> is substantially identical with beam collector incident light <b>324</b> from FIG. <b>3</b>. As a result, beam collector <b>1126</b> refracts and focuses beam collector incident light <b>1124</b> into focused light spot <b>1128</b> which is substantially identical to focused light spot <b>328</b> of FIG. <b>3</b>.

Reflected diffraction beam <b>1124</b>, which is comprised in collector incident radiation <b>1124</b> forms an angle χ(λ_L) with the normal of guiding dispersion unit <b>1160</b>, where

χ(λ_L)=θ₀.

As indicated by this formula, angle χ(λ_L) does not exhibit any dependence on wavelength or on the position of rotatable unit <b>1114</b>, but is instead fully determined by initial selection and alignment of the components of dual-beam laser system <b>1100</b>. To ensure full coupling of focused light spot <b>1128</b> into optical coupling device <b>1130</b>, the area of optical coupling device <b>1130</b> must be larger than the effective size of focused light spot <b>1128</b> and the numerical aperture of optical coupling device <b>1130</b> must be larger than the convergence of focused light spot <b>1128</b>. If these conditions are satisfied, proper initial design of dual-beam laser system <b>1100</b> results in continuous and stable coupling of selected focused light spot <b>1128</b> into waveguiding device <b>1132</b> with simultaneous and effective filtering of SSE and ASE background light regardless of wavelength variations in the system as a result of laser tuning.

FIG. 12 shows an alternative embodiment of the present invention. Dual-beam laser system <b>1200</b> comprises pivot <b>1202</b>, base <b>1204</b>, plane reflector <b>1206</b>, gain medium <b>1208</b>, dispersion unit <b>1210</b>, tuning reflector <b>1212</b>, rotatable unit <b>1214</b>, output laser beam <b>1216</b>, first-order diffracted radiation <b>1218</b>, reflected diffraction beam <b>1220</b>, beam collector incident light <b>1224</b>, beam collector <b>1226</b>, focused light spot <b>1228</b>, optical coupling device <b>1230</b>, waveguiding device <b>1232</b>, low noise laser beam <b>1234</b>, angularly-separated SSE <b>1236</b>, angulary-separated ASE <b>1238</b>, laser diode <b>1250</b>, collimation lens <b>1252</b> and guiding dispersion unit <b>1260</b>.

The structure of the embodiment shown in FIG. 12 is substantially identical with the structure of the embodiment shown in FIG. 11 except that the embodiment of FIG. 12 employs a laser diode <b>1250</b> to replace the combination of plane reflector <b>1106</b> and gain medium <b>1108</b> from FIG. <b>11</b> and introduces a collimation lens <b>1252</b> disposed along an optical path between laser diode <b>1250</b> and dispersion unit <b>1210</b>. Since the output of laser diode <b>1250</b> generally exhibits an undesirable elliptically-divergent shape, collimation lens <b>1252</b> is employed to collimate the light incident on dispersion unit <b>1210</b> at a grazing angle. In a preferred embodiment, both facets of collimation lens <b>1252</b> are treated with an anti-reflection coating to reduce light feedback from internal reflection within collimation lens <b>1252</b>. Further, a facet of laser diode <b>1250</b> oriented towards dispersion unit <b>1210</b> is also treated with an anti-reflection coating to maximize power output of laser diode <b>1250</b>.

In operation, the embodiment of FIG. 12 functions substantially the same as the embodiment of FIG. 11 because the light beam generated by laser diode <b>1250</b> in conjunction with collimation lens <b>1252</b> is substantially identical with the light beam produced by gain element <b>1108</b> and plane reflector <b>1106</b>. Consequently, the light incident at a grazing angle on diffraction grid <b>1210</b> is substantially identical with the light incident at a grazing angle on diffraction grid <b>1110</b>, and therefore the description provided for the embodiment of FIG. 11 also applies to the embodiment of FIG. <b>12</b>.

FIG. 13 shows yet another embodiment of the present invention. The preceding description of various embodiments of this invention taught how an optical aperture coupled to a waveguiding device can be employed as a spatial narrow band-pass filter to suppress SSE and ASE background light with wavelengths other than a desired wavelength. The embodiment of FIG. 13 illustrates how an optical transmission pinhole can be employed as a narrow band-pass filter to either replace or supplement and enhance the filtering effect of an optical coupling device.

FIG. 13 shows a simplified representation of the complete laser system described in prior embodiments. Laser system <b>1300</b> includes light generator <b>1370</b>, pinhole incident light <b>1372</b>, pinhole <b>1374</b>, beam collector <b>1326</b>, focused light spot <b>1328</b>, optical coupling device <b>1330</b> and waveguiding device <b>1332</b>.

Light generator <b>1370</b> and optical coupling device <b>1330</b> are mounted at opposite ends of laser system <b>1300</b>. Referring to the embodiment of FIG. 3 for example, light generator <b>1370</b> could include a subsystem comprising plane reflector <b>306</b>, gain medium <b>308</b>, dispersion unit <b>310</b>, tuning reflector <b>312</b> and guiding mirror <b>322</b>. Pinhole <b>1374</b> and beam collector <b>1326</b> are disposed along an optical path between light generator <b>1370</b> and optical coupling device <b>1330</b> such that optical aperture <b>1330</b> is located distally from light generator <b>1370</b> with respect to pinhole <b>1374</b>. In an alternative embodiment, beam collector <b>1326</b> could be replaced by a concave mirror, as disclosed in the embodiment shown in FIG. 7 for example. Optical aperture <b>1330</b> is operationally connected to waveguiding device <b>1332</b> to permit coupling of light.

In operation, light generator <b>1370</b> projects pinhole incident light <b>1372</b> towards pinhole <b>1374</b>. Pinhole <b>1374</b> includes a transparent area which permits part of pinhole incident light <b>1372</b> to propagate beyond pinhole <b>1374</b> and illuminate beam collector <b>1326</b>. Beam collector <b>1326</b> focuses the incident light radiation into focused light spot <b>1328</b> which is pre-aligned with optical coupling device <b>1330</b> to permit efficient light coupling into waveguiding device <b>1332</b>. Pinhole <b>1374</b> acts as a physical spatial narrow band-pass filter effectively suppressing SSE and ASE background light, and therefore provides a first-order filtering stage for light propagating towards beam collector <b>1326</b>. Consequently, the light incident on beam collector <b>1326</b> is already filtered prior to being concentrated into focused light spot <b>1328</b>. This advantage could be employed, among others, to relax the design constraints imposed on the optical characteristics of beam collector <b>1326</b> and the alignment requirements associated with narrow band-pass filtering as taught by the present invention. Further, the SSE and ASE radiation may be cut off by the spatial filter so that a broader optical coupling device <b>1330</b> is still adequate to couple the laser beam into waveguiding device <b>1332</b>. A larger receiving aperture decreases the probability that optical coupling device <b>1330</b> is damaged by the heat produced by the high optical energy being coupled into waveguiding device <b>1332</b>.

FIG. 14 shows yet another embodiment of the present invention. Dual-beam laser system <b>1400</b> comprises pivot <b>1402</b>, base <b>1404</b>, plane reflector <b>1406</b>, gain medium <b>1408</b>, dispersion unit <b>1410</b>, tuning reflector <b>1412</b>, rotatable unit <b>1414</b>, output laser beam <b>1416</b>, first-order diffracted radiation <b>1418</b>, reflected diffraction beam <b>1420</b>, guiding mirror <b>1422</b>, collector incident light <b>1424</b>, beam collector <b>1426</b>, focused light spot <b>1428</b>, optical coupling device <b>1430</b>, waveguiding device <b>1432</b>, angularly-separated SSE <b>1436</b>, angularly-separated ASE <b>1438</b>, and low noise laser beam <b>1434</b>.

The structure of the embodiment shown in FIG. 14 is substantially identical with the structure of the embodiment shown in FIG. 3, except that the relative position of a number of elements is changed in FIG. <b>14</b>. Specifically, beam collector <b>1426</b> is mechanically coupled to rotatable unit <b>1414</b> proximally to pivot <b>1402</b> with respect to tuning reflector <b>1412</b>. In contrast, in the embodiment of FIG. 3, beam collector <b>326</b> is mechanically coupled to rotatable unit <b>314</b> distally from tuning reflector <b>312</b> with respect to pivot <b>302</b>. Additionally, in the embodiment of FIG. 14, dispersion unit <b>1410</b> is disposed between gain medium <b>1408</b> and pivot <b>1402</b>, whereas in the embodiment of FIG. 3 gain medium <b>308</b> and pivot <b>302</b> are collocated on the same side of dispersion unit <b>310</b>. Further, in the embodiment of FIG. 14, guiding mirror <b>1422</b> is disposed between rotatable unit <b>1414</b> and gain element <b>1408</b>, whereas in the embodiment of FIG. 3, both rotatable unit <b>314</b> and gain medium <b>308</b> are collocated on the same side of guiding mirror <b>322</b>.

In operation, the embodiment of FIG. 14 functions substantially the same as the embodiment of FIG. 3 except for certain differences associated with the topographical modifications described above. For example, in the embodiment of FIG. 14, the laser cavity of dual-beam laser system <b>1400</b> is formed by a feedback path defined by plane reflector <b>1406</b>, gain medium <b>1408</b>, dispersion unit <b>1410</b>, guiding mirror <b>1422</b> and tuning reflector <b>1410</b> and is denoted as M<b>1</b>-G—M<b>2</b>-M<b>3</b>. In contrast, the corresponding feedback path in FIG. 3 does not include guiding mirror <b>1422</b>.

FIG. 15 provides a simplified schematic diagram for the embodiment shown in FIG. 14 together with a number of geometrical relationships existing between various elements of that embodiment. Dual-beam laser system <b>1500</b> comprises pivot <b>1502</b>, base <b>1504</b>, plane reflector <b>1506</b>, gain medium <b>1508</b>, dispersion unit <b>1510</b>, tuning reflector <b>1512</b>, rotatable unit <b>1514</b>, output laser beam <b>1516</b>, first-order diffracted radiation <b>1518</b>, reflected diffraction beam <b>1520</b> and guiding mirror <b>1522</b>.

The X-Y coordinate system in FIG. 15 is defined such that the Y-axis coincides with the normal of dispersion unit <b>1510</b> and points away from dispersion unit <b>1510</b>, while the X-axis lies in the plane of the diffracting surface of dispersion unit <b>1510</b>. The origin of the X-Y coordinate space is denoted at point G. The center of tuning reflector <b>1512</b> is denoted as point T.

Pivot <b>1502</b> is denoted as point O and has X-Y coordinates (x₀,y₀). The distance between points O and G is denoted L₀. Similarly, the distance between points S and G is denoted L_d. Further, the distance between points O and T is denoted as L_R.

For mode-hop-free laser tuning while rotatable unit <b>1514</b> pivots around pivot <b>1502</b>, the total length L(λ) of laser cavity M<b>1</b>-G—M<b>2</b>-M<b>3</b> must stay constant over the whole range of tunable wavelengths and must be an integer multiple of the mode number, i.e.

<maths><formula-text><i>L</i>(λ)=<i>Nλ</i>/2.</formula-text></maths>

The laser wavelength λ must satisfy the m^th-order diffraction equation for dispersion unit <b>1510</b>,

<maths><formula-text><i>mλ−d</i>[sin θ(λ)+sin θ₀],</formula-text></maths>

where θ₀represents the angle of incidence of the laser beam generated by gain medium <b>1508</b> and plane reflector <b>1506</b> on dispersion unit <b>1510</b> and d represents the spatial grating period of dispersion unit <b>1510</b>.

For a laser wavelength λ, the total cavity length L(λ) can be expressed as

<maths><formula-text><i>L</i>(λ)=|{overscore (<i>M</i>₁<i>G</i>)}|+|{overscore (<i>GM</i>₂)}|+|{overscore (<i>M</i>₂<i>M</i>₃)}|+[<i>n</i>₁(λ)−1<i>]d</i>₁,</formula-text></maths>

A portion of the M<b>1</b>-G optical path included in this equation is located inside gain medium <b>1508</b>, whose optical index or dispersion figure is n₁(λ).

In an alternative embodiment of the present invention, tuning reflector <b>1512</b> may be replaced by a Porro prism. In still another embodiment of this invention, a laser diode and collimation lens may replace plane reflector <b>1506</b> and gain medium <b>1508</b> as described, for example, in connection with the embodiment shown in FIG. <b>6</b>. Both of these two alternative embodiments are substantially identical with the embodiment of FIG. 14 from a functional point of view, except that the total cavity length L(λ) may vary due to additional dispersion introduced into the optical system.

In general, medium dispersion is a function of light wavelength and may be expressed as,

<maths><formula-text><i>n</i>₁(λ)=<i>n</i>₁₀+α₁λ+α₂λ²+α₃λ³+. . . ,</formula-text></maths>

where n₁₀represents a constant and a1, a2, . . . represent coefficients associated with higher orders of dispersion. Generally,

<maths><formula-text><i>n</i>₁₀&gt;&gt;α₁λ&gt;&gt;α₂λ²&gt;&gt;α₃λ³. . . </formula-text></maths>

Taking into consideration the dispersion introduced by gain medium <b>1508</b> and by any other dispersion-inducing elements present in the embodiment of FIG. 15, the mode number N may be expressed as, <maths><math><mtable><mtr><mtd><mrow><mi>N</mi><mo>=</mo><mstyle><mtext> </mtext></mstyle><mo></mo><mrow><mrow><mn>2</mn><mo></mo><mi>m</mi><mo></mo><mrow><mo>{</mo><mfrac><mrow><mrow><mrow><mo>(</mo><mrow><msub><mi>L</mi><mi>o</mi></msub><mo>+</mo><mrow><msub><mi>L</mi><mi>d</mi></msub><mo></mo><mi>cos</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>α</mi></mrow></mrow><mo>)</mo></mrow><mo></mo><mrow><mi>cos</mi><mo></mo><mrow><mo>(</mo><mrow><mi>α</mi><mo>+</mo><mrow><mn>2</mn><mo></mo><mi>Δ</mi></mrow></mrow><mo>)</mo></mrow></mrow></mrow><mo>+</mo><mrow><msub><mi>L</mi><mi>d</mi></msub><mo></mo><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><mrow><mi>sin</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>α</mi><mo></mo><mrow><mo></mo><mrow><mi>sin</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>Δ</mi></mrow><mo></mo></mrow></mrow></mrow><mo>)</mo></mrow></mrow><mo>+</mo><mrow><msub><mi>a</mi><mn>1</mn></msub><mo></mo><msub><mi>d</mi><mn>1</mn></msub></mrow></mrow><mi>d</mi></mfrac><mo>}</mo></mrow></mrow><mo>+</mo></mrow></mrow></mtd></mtr><mtr><mtd><mrow><mstyle><mtext> </mtext></mstyle><mo></mo><mrow><mn>2</mn><mo></mo><mi>m</mi><mo></mo><mrow><mo>{</mo><mrow><mfrac><mrow><mrow><mrow><mo>(</mo><mrow><msub><mi>L</mi><mi>o</mi></msub><mo>+</mo><mrow><msub><mi>L</mi><mi>d</mi></msub><mo></mo><mi>cos</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>α</mi></mrow></mrow><mo>)</mo></mrow><mo></mo><mrow><mi>sin</mi><mo></mo><mrow><mo>(</mo><mrow><mi>α</mi><mo>+</mo><mrow><mn>2</mn><mo></mo><mi>Δ</mi></mrow></mrow><mo>)</mo></mrow></mrow></mrow><mo>-</mo><mstyle>&lt;mtext /&gt;</mstyle><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mrow><msub><mi>L</mi><mi>d</mi></msub><mo></mo><mi>sin</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>αcos</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>Δ</mi></mrow></mrow><mi>λ</mi></mfrac><mo>·</mo></mrow></mrow></mrow></mrow></mtd></mtr><mtr><mtd><mrow><mrow><mstyle><mtext> </mtext></mstyle><mo></mo><mrow><msqrt><mrow><mn>1</mn><mo>-</mo><msup><mrow><mo>(</mo><mrow><mfrac><mi>λ</mi><mi>d</mi></mfrac><mo></mo><mi>sin</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><msub><mi>θ</mi><mn>0</mn></msub></mrow><mo>)</mo></mrow><mn>2</mn></msup></mrow></msqrt><mo>+</mo><mrow><mo>(</mo><mrow><mrow><msub><mi>a</mi><mn>2</mn></msub><mo></mo><mi>λ</mi></mrow><mo>+</mo><mrow><msub><mi>a</mi><mn>3</mn></msub><mo></mo><msup><mi>λ</mi><mn>2</mn></msup></mrow><mo>+</mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>…</mi></mrow><mo></mo><mstyle><mtext> </mtext></mstyle><mo>)</mo></mrow></mrow><mo>}</mo></mrow><mo>+</mo></mrow></mtd></mtr><mtr><mtd><mrow><mstyle><mtext> </mtext></mstyle><mo></mo><mrow><mn>2</mn><mo></mo><mi>m</mi><mo></mo><mrow><mo>{</mo><mfrac><mrow><mrow><mo></mo><mover><mrow><msub><mi>M</mi><mn>1</mn></msub><mo></mo><mi>G</mi></mrow><mi>_</mi></mover><mo></mo></mrow><mo>+</mo><mrow><mrow><mo>(</mo><mrow><msub><mi>n</mi><mn>10</mn></msub><mo>-</mo><mn>1</mn></mrow><mo>)</mo></mrow><mo></mo><msub><mi>d</mi><mn>1</mn></msub></mrow><mo>-</mo><mrow><msub><mi>L</mi><mi>R</mi></msub><mo></mo><mi>sin</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>β</mi></mrow><mo>-</mo><mrow><mrow><mo>[</mo><mrow><mrow><msub><mi>L</mi><mi>d</mi></msub><mo></mo><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><mrow><mi>sin</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>α</mi><mo></mo><mrow><mo></mo><mrow><mi>sin</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>Δ</mi></mrow><mo></mo></mrow></mrow></mrow><mo>)</mo></mrow></mrow><mo>+</mo><mstyle>&lt;mtext /&gt;</mstyle><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mrow><mrow><mo>(</mo><mrow><msub><mi>L</mi><mi>o</mi></msub><mo>+</mo><mrow><msub><mi>L</mi><mi>d</mi></msub><mo></mo><mi>cos</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>α</mi></mrow></mrow><mo>)</mo></mrow><mo></mo><mrow><mi>cos</mi><mo></mo><mrow><mo>(</mo><mrow><mi>α</mi><mo>+</mo><mrow><mn>2</mn><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>Δ</mi></mrow></mrow><mo>)</mo></mrow></mrow></mrow></mrow><mo>]</mo></mrow><mo></mo><mi>sin</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><msub><mi>θ</mi><mn>0</mn></msub></mrow></mrow><mi>λ</mi></mfrac><mo>}</mo></mrow></mrow></mrow></mtd></mtr></mtable></math><img id="EMI-M00005" file="US06606340-20030812-M00005.TIF" img-content="math" img-format="tif" alt="embedded image" /><attachments><attachment idref="MATHEMATICA-00005" attachment-type="nb" file="US06606340-20030812-M00005.NB" /></attachments></maths>

Parameters {overscore (M₁G)}, L₀,L_d,L_R, α, β, Δ(Δ=ψ₀−90°) are only dependent on the physical design and static setup of dual-beam laser system <b>1500</b> and do not vary as rotatable unit <b>1514</b> pivots around pivot <b>1502</b>. Consequently, for a laser wavelength λ, the cavity mode number N may be expressed as,

<maths><formula-text><i>N=N</i>₀<i>+ΔN</i>₀<i>+ΔN</i>(λ),</formula-text></maths>

where the cavity constant mode number N₀is given by, <maths><math><mrow><msub><mi>N</mi><mn>0</mn></msub><mo>=</mo><mrow><mn>2</mn><mo></mo><mi>m</mi><mo></mo><mrow><mrow><mo>{</mo><mfrac><mrow><mrow><mrow><mo>(</mo><mrow><msub><mi>L</mi><mi>o</mi></msub><mo>+</mo><mrow><msub><mi>L</mi><mi>d</mi></msub><mo></mo><mi>cos</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>α</mi></mrow></mrow><mo>)</mo></mrow><mo></mo><mrow><mi>cos</mi><mo></mo><mrow><mo>(</mo><mrow><mi>α</mi><mo>+</mo><mrow><mn>2</mn><mo></mo><mi>Δ</mi></mrow></mrow><mo>)</mo></mrow></mrow></mrow><mo>+</mo><mrow><msub><mi>L</mi><mi>d</mi></msub><mo></mo><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><mrow><mi>sin</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>α</mi><mo></mo><mrow><mo></mo><mrow><mi>sin</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>Δ</mi></mrow><mo></mo></mrow></mrow></mrow><mo>)</mo></mrow></mrow><mo>+</mo><mrow><msub><mi>a</mi><mn>1</mn></msub><mo></mo><msub><mi>d</mi><mn>1</mn></msub></mrow></mrow><mi>d</mi></mfrac><mo>}</mo></mrow><mo>.</mo></mrow></mrow></mrow></math><img id="EMI-M00006" file="US06606340-20030812-M00006.TIF" img-content="math" img-format="tif" alt="embedded image" /><attachments><attachment idref="MATHEMATICA-00006" attachment-type="nb" file="US06606340-20030812-M00006.NB" /></attachments></maths>

The mode shift ΔN₀induced by initial alignment and setup of dual-beam laser system <b>1500</b> may be expressed as, <maths><math><mtable><mtr><mtd><mrow><mrow><mi>Δ</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mrow><msub><mi>N</mi><mn>0</mn></msub><mo></mo><mrow><mo>(</mo><mi>λ</mi><mo>)</mo></mrow></mrow></mrow><mo>=</mo><mstyle><mtext> </mtext></mstyle><mo></mo><mrow><mrow><mn>2</mn><mo></mo><mi>m</mi><mo></mo><mrow><mo>{</mo><mfrac><mrow><mrow><mo></mo><mover><mrow><msub><mi>M</mi><mn>1</mn></msub><mo></mo><mi>G</mi></mrow><mi>_</mi></mover><mo></mo></mrow><mo>+</mo><mrow><mrow><mo>(</mo><mrow><msub><mi>n</mi><mn>10</mn></msub><mo>-</mo><mn>1</mn></mrow><mo>)</mo></mrow><mo></mo><msub><mi>d</mi><mn>1</mn></msub></mrow><mo>-</mo><mrow><msub><mi>L</mi><mi>R</mi></msub><mo></mo><mi>sin</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>β</mi></mrow><mo>-</mo><mrow><mrow><mo>[</mo><mrow><mrow><msub><mi>L</mi><mi>d</mi></msub><mo></mo><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><mrow><mi>sin</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>α</mi><mo></mo><mrow><mo></mo><mrow><mi>sin</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>Δ</mi></mrow><mo></mo></mrow></mrow></mrow><mo>)</mo></mrow></mrow><mo>+</mo><mstyle>&lt;mtext /&gt;</mstyle><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mrow><mrow><mo>(</mo><mrow><msub><mi>L</mi><mi>o</mi></msub><mo>+</mo><mrow><msub><mi>L</mi><mi>d</mi></msub><mo></mo><mi>cos</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>α</mi></mrow></mrow><mo>)</mo></mrow><mo></mo><mrow><mi>cos</mi><mo></mo><mrow><mo>(</mo><mrow><mi>α</mi><mo>+</mo><mrow><mn>2</mn><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>Δ</mi></mrow></mrow><mo>)</mo></mrow></mrow></mrow></mrow><mo>]</mo></mrow><mo></mo><mi>sin</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>θ</mi></mrow></mrow><mi>λ</mi></mfrac><mo>}</mo></mrow></mrow><mo>+</mo></mrow></mrow></mtd></mtr><mtr><mtd><mrow><mstyle><mtext> </mtext></mstyle><mo></mo><mrow><mn>2</mn><mo></mo><mi>m</mi><mo></mo><mrow><mo>{</mo><mrow><mfrac><mrow><mrow><mrow><mo>(</mo><mrow><msub><mi>L</mi><mi>o</mi></msub><mo>+</mo><mrow><msub><mi>L</mi><mi>d</mi></msub><mo></mo><mi>cos</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>α</mi></mrow></mrow><mo>)</mo></mrow><mo></mo><mrow><mi>sin</mi><mo></mo><mrow><mo>(</mo><mrow><mi>α</mi><mo>+</mo><mrow><mn>2</mn><mo></mo><mi>Δ</mi></mrow></mrow><mo>)</mo></mrow></mrow></mrow><mo>-</mo><mstyle>&lt;mtext /&gt;</mstyle><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mrow><msub><mi>L</mi><mi>d</mi></msub><mo></mo><mi>sin</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>αcos</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>Δ</mi></mrow></mrow><mi>λ</mi></mfrac><mo>·</mo><msqrt><mrow><mn>1</mn><mo>-</mo><msup><mrow><mo>(</mo><mrow><mfrac><mi>λ</mi><mi>d</mi></mfrac><mo></mo><mi>sin</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><msub><mi>θ</mi><mn>0</mn></msub></mrow><mo>)</mo></mrow><mn>2</mn></msup></mrow></msqrt></mrow><mo>}</mo></mrow></mrow></mrow></mtd></mtr></mtable></math><img id="EMI-M00007" file="US06606340-20030812-M00007.TIF" img-content="math" img-format="tif" alt="embedded image" /><attachments><attachment idref="MATHEMATICA-00007" attachment-type="nb" file="US06606340-20030812-M00007.NB" /></attachments></maths>

Analogously, the cavity mode shift ΔN(λ) induced by dispersion associated with the optical components of dual-beam laser system <b>1500</b> is provided by,

<maths><formula-text><i>ΔN</i>(λ)=2<i>m</i>(α₂λ+α₃λ²+. . . ):</formula-text></maths>

Mode-hop-free laser tuning over the entire tuning band of gain medium <b>1508</b> and dispersion unit <b>1510</b> can be achieved only when

<maths><formula-text><i>ΔN</i>₀(λ)+Δ<i>N</i>(λ)=0.</formula-text></maths>

This equation suggests that if dual-beam laser system <b>1500</b> exhibits sufficiently-high nonlinear dispersion or misalignment, mode-hop-free laser tuning might not be achievable. Mode-hop-free tuning may only be maintained if the ΔN₀(λ)+ΔN(λ)&lt;&lt;1 over the entire tuning range.

An advantage of the present invention is that mode-hop free tuning can be achieved by proper selection of certain parameters during the design of dual-beam laser system <b>1500</b> such as the position of plane reflector <b>1506</b> (|{overscore (M₁G)}|), the position of pivot <b>1502</b> (L₀,α), the position of guiding mirror <b>1522</b> (L_d), or the location of tuning reflector <b>1512</b> (L_R,β). Adjustment of any combination of these parameters can provide the necessary condition, ΔN₀(λ)+ΔN(λ)&lt;&lt;1.

For example, the position of tuning reflector <b>1512</b> may be selected such that, |{overscore (M₁G)}|+(n₁₀−1)d₁−(L_dsin ψ₀+L₀cos α+L_Rsin β)=0. At the same time, the position of pivot <b>1502</b> can be defined such that the mode shift ΔN₀(λ) compensates the dispersion shift up to high orders. For a practical device, the compensation of high order dispersion yields and guarantees the continues tuning of the laser from the grating-tuned external cavity, i.e. ΔN₀(λ)+ΔN(λ)&lt;&lt;1.

The following discussion provides a mathematical description for the structure and operation of the embodiment shown in FIG. <b>14</b>. The intensity of collector incident light <b>1424</b> in the X′-Y′ focal plane of beam collector <b>1426</b> is described by a two-dimensional (x′, y′) equation which includes an angular-cone distribution-function representing the beam focusing effect of beam collector <b>1426</b>,

<i>I</i>(λ,<i>x′,y</i>′)=<i>I</i>(λ)ζ(<i>x′−x′</i>_λ<i>−y′,y′</i>_λ,ƒ,Ω),

where a normalized arbitrary distribution function ζ(x′−x′_λ,y′−y′_λ,ƒ,Ω) provides a light intensity distribution for focused light spot <b>1428</b> with beam center (x_λ, y_λ, f) in the X′-Y′ focal plane of beam collector <b>1426</b> as illustrated in FIG. <b>16</b>.

If the grating of diffraction grid <b>1410</b> exhibits a one-dimensional variation, coordinates x′_λand y′_λcan be expressed as x′_λ=ƒ tan(θ(λ)−θ(λ_L)) and y′_λ=0 where f represents the focal length of the beam collector <b>1426</b>. In a preferred embodiment, beam collector <b>1426</b> comprises a convex lens and f represents the focal length of the convex lens. In the X-Y focal plane of beam collector <b>1426</b>, optical coupling device <b>1430</b> is aligned with the center of focused light spot <b>1428</b> to receive the light with laser wavelength λ_L. In a preferred embodiment, optical coupling device <b>1430</b> comprises a single-mode fiber, a multi-mode transparent fiber or a waveguide designed for coupling a light beam. Through proper alignment with focused light spot <b>1428</b>, optical coupling device <b>1430</b> receives only light with wavelength λ_Lby filtering out light with other wavelengths than λ_L.

The light energy E(λ) coupled into waveguiding device <b>1432</b> by optical coupling device <b>1430</b> is determined by convolution of the X-Y focal light intensity distribution function I(λ,x′,y′) with both an optical aperture function ρ(x′−x′_λ_{<sub2>L</sub2>},y′−y′_λ_{<sub2>L</sub2>},z′−ƒ,Ω) characteristic to optical coupling device <b>1430</b> and with a numerical aperture function κ(λ) representing the average insertion or surface-reflection light coupling loss over the entire area of optical coupling device <b>1430</b>,

<maths><formula-text><i>E</i>^(fiber)(λ)=<i>I</i>(λ)(1−κ(λ))∫∫∫ρ(<i>x′−x′</i>_λ_{<sub2>L</sub2>}<i>,y′−y′</i>_λ_{<sub2>L</sub2>},ƒ,Ω)·ζ(<i>x′−x′</i>_λ<i>,y′−y′</i>_λ,ƒ,Ω)<i>dx′dy′dΩ=E</i>(λ)(1−κ(λ)σ(λ)</formula-text></maths>

where the filtering effect of the beam coupling device is represented by a filtering function

<maths><formula-text>σ(λ)=∫∫∫ρ(<i>x′−x′</i>_λ_{<sub2>L</sub2>}<i>,y′−y′</i>_λ_{<sub2>L</sub2>},ƒ,Ω)·ζ(<i>x′−x′</i>_λ<i>,y′−y′</i>_λ,ƒ,Ω)<i>dx′dy′dΩ.</i></formula-text></maths>

If the numerical aperture of optical coupling device <b>1430</b> is larger than the divergence of focused light spot <b>1428</b> and if the area of optical coupling device <b>1430</b> is larger than the size of focused light spot <b>1428</b> at the laser wavelength λ=λ_L, all the light of the respective beam spot is coupled into waveguiding device <b>1432</b>. The energy of the light coupled into the fiber can therefore be expressed as,

<maths><formula-text><i>E</i>^(fiber)(λ)≈<i>E</i>(λ_L)(1−κ(λ_L)).</formula-text></maths>

FIG. 16 illustrates how the present invention achieves spatial narrow band-pass filtering for light coupled into waveguiding device <b>1432</b> of FIG. <b>14</b>. FIG. 16 shows the focal plane <b>1600</b> of beam collector <b>1426</b>. Focal plane <b>1600</b> includes an aligned light spot <b>1602</b>, a misaligned light spot <b>1604</b> and an optical aperture <b>1606</b>. Consistent with the previous discussion, beam collector <b>1426</b> refracts collector incident light <b>1424</b> and concentrates it into a number of discrete light spots in the focal plane <b>1600</b> of beam collector <b>1426</b>. The spatial position of each light spot depends upon the wavelength of the light associated with that particular light spot.

Aligned light spot <b>1602</b> represents focused light spot <b>1428</b> from FIG. 14 which comprises light of substantially wavelength λ_L. Aligned light spot <b>1602</b> is centered at coordinates (x_λ_{<sub2>L</sub2>}, y_λ_{<sub2>L</sub2>}) and has a radius of r_λ_{<sub2>L</sub2>}. The radius r_λ_{<sub2>L </sub2>}is selected such that the resulting circular area includes only light with an intensity of at least 1/e of the peak value existing within aligned light spot <b>1602</b>. Aligned light spot <b>1602</b> is concentrically collocated with optical aperture <b>1606</b>. If the radius r₀of optical aperture <b>1606</b> is larger than the radius r^λ_{<sub2>L </sub2>}of aligned light spot <b>1602</b>, aligned light spot <b>1602</b> is fully contained within optical aperture <b>1606</b> and light from aligned light spot <b>1602</b> may be fully coupled into waveguiding device <b>1432</b>. In contrast, misaligned light beam <b>1604</b> is centered at coordinates (x_λ, y_λ) and is not fully contained within optical aperture <b>1606</b>. Consequently, light associated with misaligned light spot <b>1604</b> cannot be fully coupled into waveguiding device <b>1432</b> and is therefore at least partially filtered out.

As a result of spatial narrow band-pass filtering, therefore, for both light and SSE and ASE background noise radiation with wavelengths other than λ_L(λ≠λ_L), the energy coupled into waveguiding device <b>1432</b> upon proper alignment of optical aperture <b>1606</b> with aligned light spot <b>1602</b> is minimized such that E^(fiber)(λ)≈0.

Referring to FIG. 16, for r_λ≦r₀, the filtering function associated with optical aperture <b>1406</b> can be expressed as

<maths><formula-text>σ(λ)=π⁻¹<i>r</i>_λ⁻²·∫∫∫ρ(<i>x′−x′</i>_λ_{<sub2>L</sub2>}<i>,y′−y′</i>_λ_{<sub2>L</sub2>},ƒ,Ω)·ζ(<i>x′−x′</i>_λ<i>,y′−y′</i>_λ,ƒ,Ω)<i>dx′dy′dΩ.</i></formula-text></maths>

For r_λ≧r₀, however, the filtering function can be represented by

<maths><formula-text>σ(λ)=π⁻¹<i>r</i>_λ_{<sub2>L</sub2>}⁻²·∫∫∫ρ(<i>x′−x′</i>_λ_{<sub2>L</sub2>}<i>,y′−y′</i>_λ_{<sub2>L</sub2>},ƒ,Ω)·ζ(<i>x′−x′</i>_λ<i>,y′−y′</i>_λ,ƒ,Ω)<i>dx′dy′dΩ.</i></formula-text></maths>

The optical aperture function of optical aperture <b>1606</b> and the normalized distribution function describing the light intensity distribution for aligned light spot <b>1602</b> can then be approximated by,

ρ(<i>x′−x′</i>_λ_{<sub2>L</sub2>}<i>,y′−y′</i>_λ_{<sub2>L</sub2>},ƒ,Ω)≡μ(<i>x′−x′</i>_λ_{<sub2>L</sub2>}<i>,y′−y′</i>_λ_{<sub2>L</sub2>},ƒ)Θ(Ω)Γ(<i>r</i>_λ_{<sub2>L</sub2>}<i>−r</i>),

and respectively,

<maths><formula-text>ζ(<i>x′−x′</i>_λ<i>,y′−y′</i>_λ,ƒ,Ω)≡τ(<i>x′−x′</i>_λ<i>,y′−y′</i>_λ,ƒ)Θ_FIBER(Ω)Γ(<i>r</i>_λ<i>−r</i>′),</formula-text></maths>

where Ω_Lrepresents the cone angle of the light intensity distribution of aligned light spot <b>1602</b>, Ω_FIBERrepresents the numerical aperture of optical aperture <b>1606</b>, and the following formulas apply:

<maths><formula-text><i>r</i>={square root over ((<i>x′−x′</i>_λ_{<sub2>L</sub2>})²+(<i>y′−y′</i>_λ_{<sub2>L</sub2>})²)};</formula-text></maths>

<maths><formula-text><i>r</i>={square root over ((<i>x′−x′</i>_λ)²+(<i>y′−y′</i>_λ)²)};</formula-text></maths>

<maths><math><mrow><mrow><mrow><mi>Γ</mi><mo></mo><mrow><mo>(</mo><mrow><msub><mi>r</mi><msub><mi>λ</mi><mi>L</mi></msub></msub><mo>-</mo><mi>r</mi></mrow><mo>)</mo></mrow></mrow><mo>=</mo><mrow><mo>{</mo><mtable><mtr><mtd><mn>1</mn></mtd><mtd><mrow><mrow><msub><mi>r</mi><msub><mi>λ</mi><mi>L</mi></msub></msub><mo>-</mo><mi>r</mi></mrow><mo>≥</mo><mn>0</mn></mrow></mtd></mtr><mtr><mtd><mn>0</mn></mtd><mtd><mrow><mrow><msub><mi>r</mi><msub><mi>λ</mi><mi>L</mi></msub></msub><mo>-</mo><mi>r</mi></mrow><mo>&lt;</mo><mn>0</mn></mrow></mtd></mtr></mtable><mo>}</mo></mrow></mrow><mo>;</mo></mrow></math><math><mrow><mrow><mrow><mi>Γ</mi><mo></mo><mrow><mo>(</mo><mrow><msub><mi>r</mi><mi>λ</mi></msub><mo>-</mo><mi>r</mi></mrow><mo>)</mo></mrow></mrow><mo>=</mo><mrow><mo>{</mo><mtable><mtr><mtd><mn>1</mn></mtd><mtd><mrow><mrow><msub><mi>r</mi><mi>λ</mi></msub><mo>-</mo><mi>r</mi></mrow><mo>≥</mo><mn>0</mn></mrow></mtd></mtr><mtr><mtd><mn>0</mn></mtd><mtd><mrow><mrow><msub><mi>r</mi><mi>λ</mi></msub><mo>-</mo><mi>r</mi></mrow><mo>&lt;</mo><mn>0</mn></mrow></mtd></mtr></mtable><mo>}</mo></mrow></mrow><mo>;</mo></mrow></math><img id="EMI-M00008" file="US06606340-20030812-M00008.TIF" img-content="math" img-format="tif" alt="embedded image" /><attachments><attachment idref="MATHEMATICA-00008" attachment-type="nb" file="US06606340-20030812-M00008.NB" /></attachments></maths><maths><math><mrow><mrow><mi>Θ</mi><mo></mo><mrow><mo>(</mo><mi>Ω</mi><mo>)</mo></mrow></mrow><mo>≈</mo><mrow><mo>{</mo><mtable><mtr><mtd><mn>1</mn></mtd><mtd><mrow><mi>Ω</mi><mo>≤</mo><msub><mi>Ω</mi><mi>L</mi></msub></mrow></mtd></mtr><mtr><mtd><mn>0</mn></mtd><mtd><mrow><mi>Ω</mi><mo>&gt;</mo><msub><mi>Ω</mi><mi>L</mi></msub></mrow></mtd></mtr></mtable><mo>}</mo></mrow></mrow></math><img id="EMI-M00009" file="US06606340-20030812-M00009.TIF" img-content="math" img-format="tif" alt="embedded image" /><attachments><attachment idref="MATHEMATICA-00009" attachment-type="nb" file="US06606340-20030812-M00009.NB" /></attachments></maths>

(This formula represents the angular distribution function of aligned light spot <b>1602</b>); <maths><math><mrow><mrow><msub><mi>Θ</mi><mi>FIBER</mi></msub><mo></mo><mrow><mo>(</mo><mi>Ω</mi><mo>)</mo></mrow></mrow><mo>≈</mo><mrow><mo>{</mo><mtable><mtr><mtd><mn>1</mn></mtd><mtd><mrow><mi>Ω</mi><mo>≤</mo><msub><mi>Ω</mi><mi>FIBER</mi></msub></mrow></mtd></mtr><mtr><mtd><mn>0</mn></mtd><mtd><mrow><mi>Ω</mi><mo>&gt;</mo><msub><mi>Ω</mi><mi>FIBER</mi></msub></mrow></mtd></mtr></mtable><mo>}</mo></mrow></mrow></math><img id="EMI-M00010" file="US06606340-20030812-M00010.TIF" img-content="math" img-format="tif" alt="embedded image" /><attachments><attachment idref="MATHEMATICA-00010" attachment-type="nb" file="US06606340-20030812-M00010.NB" /></attachments></maths>

(This formula represents the numerical function of optical aperture <b>1606</b>);

<maths><formula-text><i>x′</i>_λ=ƒ tan[θ(λ)−θ(λ_L)];</formula-text></maths>

<maths><formula-text><i>y′</i>_λ=0;</formula-text></maths>

<i>x′</i>_λ_{<sub2>L</sub2>}=0;

<maths><formula-text><i>y′</i>_λ_{<sub2>L</sub2>}=0.</formula-text></maths>

FIG. 17 shows a simulation of the effectiveness of SSE and ASE suppression achieved by an embodiment of the present invention for r_λ_{<sub2>L</sub2>}=r₀, ƒ=1000·r₀, d=1 μm, Ω_L≦Ω_FIBERand assuming a Gaussian light intensity distribution for aligned light spot <b>1602</b>. Over the emission band of gain medium <b>1408</b> of FIG. 14, the present invention filters out SSE and ASE background noise radiation at all wavelengths other than the desired laser wavelength λ_L, which is shown in FIG. 17 to be approximately 1.54 μm. Consequently, the only light coupled into waveguiding device <b>1432</b> is light with the desired wavelength, λ_L.

An advantage of the present invention, as illustrated in the numerous embodiments discussed herein, is that it can maintain a maximum and constant level of light coupled into waveguiding device <b>1432</b> while the wavelength of output laser beam <b>1416</b> and of low noise laser beam <b>1434</b> is tuned across a wide range of wavelengths, i.e., it provides automatic wavelength and power tracking for the low-noise output laser beam. In the embodiment shown in FIG. 14, this advantage is achieved by appropriate selection of the physical dimensions of dual-beam laser system <b>1400</b>.

FIG. 18 shows an alternative embodiment of the present invention. Dual-beam laser system <b>1800</b> comprises pivot <b>1802</b>, base <b>1804</b>, dispersion unit <b>1810</b>, tuning reflector <b>1812</b>, rotatable unit <b>1814</b>, output laser beam <b>1816</b>, first-order diffracted radiation <b>1818</b>, reflected diffraction beam <b>1820</b>, guiding mirror <b>1822</b>, collector incident light <b>1824</b>, beam collector <b>1826</b>, focused light spot <b>1828</b>, optical coupling device <b>1830</b>, waveguiding device <b>1832</b>, low noise laser beam <b>1834</b>, angularly-separated SSE <b>1836</b>, angulary-separated ASE <b>1838</b>, laser diode <b>1850</b> and collimation lens <b>1852</b>.

The structure of the embodiment shown in FIG. 18 is substantially identical with the structure of the embodiment shown in FIG. 14 except that the embodiment of FIG. 18 employs a laser diode <b>1850</b> to replace the combination of the plane reflector <b>1406</b> and the gain medium <b>1408</b> from FIG. <b>14</b> and introduces a collimation lens <b>1852</b> disposed along an optical path between laser diode <b>1850</b> and dispersion unit <b>1810</b>. Since the output of laser diode <b>1850</b> generally exhibits an undesirable elliptically-divergent shape, collimation lens <b>1852</b> is employed to collimate the light incident on dispersion unit <b>1810</b> at a grazing angle. In a preferred embodiment, both facets of collimation lens <b>1852</b> are treated with an anti-reflection coating to reduce light feedback from internal reflection within collimation lens <b>1852</b>. Further, a facet of laser diode <b>1850</b> oriented towards dispersion unit <b>1810</b> is also treated with an anti-reflection coating to maximize power output of laser diode <b>1850</b>.

In operation, the embodiment of FIG. 18 functions substantially the same as the embodiment of FIG. 14 because the light beam generated by laser diode <b>1850</b> in conjunction with collimation lens <b>1852</b> is substantially identical with the light beam produced by gain element <b>1408</b> and plane reflector <b>1406</b>. Consequently, the light incident at a grazing angle on diffraction grid <b>1810</b> is substantially identical with the light incident at a grazing angle on diffraction grid <b>1810</b>, and therefore the description provided for the embodiment of FIG. 18 generally applies to the embodiment of FIG. <b>18</b>.

FIG. 19 shows another alternative embodiment of the present invention. Dual-beam laser system <b>1900</b> comprises pivot <b>1902</b>, base <b>1904</b>, plane reflector <b>1906</b>, gain medium <b>1908</b>, dispersion unit <b>1910</b>, tuning reflector <b>1912</b>, rotatable unit <b>1914</b>, output laser beam <b>1916</b>, first-order diffracted radiation <b>1918</b>, reflected diffraction beam <b>1920</b>, guiding mirror <b>1922</b>, focused light spot <b>1928</b>, optical coupling device <b>1930</b>, waveguiding device <b>1932</b>, low noise laser beam <b>1934</b>, angularly-separated SSE <b>1936</b>, angulary-separated ASE <b>1938</b>, and concave mirror <b>1954</b>.

The structure of the embodiment of FIG. 19 is substantially identical with the structure of the embodiment of FIG. 14, except that the embodiment of FIG. 19 substitutes a concave mirror <b>1954</b> for beam collector <b>1426</b>. Concave mirror <b>1954</b> is mechanically coupled to rotatable unit <b>1914</b> such that a concave reflective surface of concave mirror <b>1954</b> is oriented in the general direction of guiding mirror <b>1922</b> to intercept reflected diffraction beam <b>1920</b>. Optical coupling device <b>1930</b> and waveguiding device <b>1932</b> are mechanically coupled to rotatable unit <b>1914</b> on the same side of concave mirror <b>1954</b> as guiding mirror <b>1922</b>.

In operation, the embodiment shown in FIG. 19 functions substantially the same as the embodiment of FIG. <b>14</b>. Reflected diffraction beam <b>1920</b> is substantially identical with collector incident light <b>1424</b> from FIG. <b>14</b>. Unlike in the embodiment of FIG. 14, however, reflected diffraction beam <b>1920</b> is not refracted by beam collector <b>1426</b>, which comprises a lens, but is instead reflected by concave mirror <b>1954</b>.

Concave mirror <b>1954</b> is designed to reflect and focus reflected diffraction beam <b>1920</b> in a pattern substantially identical with the pattern experienced by the light refracted by beam collector <b>1426</b> in the embodiment of FIG. <b>14</b>. Consequently, concave mirror <b>1954</b> reflects and concentrates reflected diffraction beam <b>1920</b> into focused light spot <b>1928</b> which is substantially identical to focused light spot <b>1428</b> of FIG. <b>14</b>. To take advantage of the spatial filtering technique previously discussed in connection with the embodiment of FIG. 14, optical coupling device <b>1930</b> is mechanically coupled to rotatable unit <b>1914</b> such that focused light spot <b>1928</b> is coupled into optical coupling device <b>1930</b>.

Upon reflection by concave mirror <b>1954</b>, reflected diffraction beam <b>1924</b> propagates at an angle ξ(λ_L) with respect to the focal axis of concave mirror <b>1954</b>. Angle ξ(λ_L) does not exhibit any dependence on wavelength or on the position of rotatable unit <b>1914</b>, but is instead fully determined by initial selection and alignment of the components of dual-beam laser system <b>1900</b>. To ensure full coupling of focused light spot <b>1928</b> into optical coupling device <b>1930</b>, the area of optical coupling device <b>1930</b> must be larger than the effective size of focused light spot <b>1928</b> and the numerical aperture of optical coupling device <b>1930</b> must be larger than the convergence of focused light spot <b>1928</b>. If these conditions are satisfied, proper initial design of dual-beam laser system <b>1900</b> results in continuous and stable coupling of selected focused light spot <b>1928</b> into waveguiding device <b>1932</b> with simultaneous and effective filtering of SSE and ASE background light regardless of wavelength variations in the system as a result of laser tuning.

FIG. 20 shows yet another embodiment of the present invention. Dual-beam laser system <b>2000</b> comprises pivot <b>2002</b>, base <b>2004</b>, dispersion unit <b>2010</b>, tuning reflector <b>2012</b>, rotatable unit <b>2014</b>, output laser beam <b>2016</b>, first-order diffracted radiation <b>2018</b>, reflected diffraction beam <b>2020</b>, guiding mirror <b>2022</b>, focused light spot <b>2028</b>, optical coupling device <b>2030</b>, waveguiding device <b>2032</b>, low noise laser beam <b>2034</b>, angularly-separated SSE <b>2036</b>, angulary-separated ASE <b>2038</b>, laser diode <b>2050</b>, collimation lens <b>2052</b> and concave mirror <b>2054</b>.

The structure of the embodiment shown in FIG. 20 is substantially identical with the embodiment shown in FIG. 14, except that the embodiment of FIG. 20 includes the modifications introduced by the embodiments shown in FIG. <b>18</b> and FIG. <b>19</b>. More specifically, the embodiment of FIG. 20 employs a laser diode <b>2050</b> to replace the combination of the plane reflector <b>1406</b> and the gain medium <b>1408</b> from FIG. <b>14</b> and introduces a collimation lens <b>2052</b> disposed along an optical path between laser diode <b>2050</b> and dispersion unit <b>2010</b>, as discussed in connection with FIG. <b>18</b>. Additionally, the embodiment of FIG. 20 substitutes a concave mirror <b>2054</b> for beam collector <b>1426</b>, as described in conjunction with FIG. <b>19</b>. In a preferred embodiment, both facets of collimation lens <b>2052</b> are treated with an anti-reflection coating to reduce light feedback from internal reflection within collimation lens <b>2052</b>. Further, a facet of laser diode <b>2050</b> oriented towards dispersion unit <b>2010</b> is also treated with an anti-reflection coating to maximize power output of laser diode <b>2050</b>.

In operation, both modifications operated to the embodiment shown in FIG. 20 perform substantially identical functions as the original elements they replace, as discussed in connection with the embodiments of FIG. <b>18</b> and FIG. <b>19</b>. Consequently, the descriptions provided for the embodiments shown in FIGS. 14, <b>18</b> and <b>19</b> also apply to the embodiment of FIG. <b>20</b>.

FIG. 21 shows yet another alternative embodiment of the present invention. Dual-beam laser system <b>2100</b> comprises pivot <b>2102</b>, base <b>2104</b>, plane reflector <b>2106</b>, gain medium <b>2108</b>, dispersion unit <b>2110</b>, tuning reflector <b>2112</b>, rotatable unit <b>2114</b>, output laser beam <b>2116</b>, first-order diffracted radiation <b>2118</b>, reflected diffraction beam <b>2120</b>, beam collector incident light <b>2124</b>, beam collector <b>2126</b>, focused light spot <b>2128</b>, optical coupling device <b>2130</b>, waveguiding device <b>2132</b>, low noise laser beam <b>2134</b>, angularly-separated SSE <b>2136</b>, angulary-separated ASE <b>2138</b>, and guiding dispersion unit <b>2160</b>.

The structure of the embodiment of FIG. 21 is substantially identical with the structure of the embodiment of FIG. 14, except that in the embodiment of FIG. 21 a guiding dispersion unit <b>2160</b> substitutes guiding mirror <b>1422</b> of FIG. <b>14</b> and beam collector <b>2126</b>, optical coupling device <b>2130</b> and waveguiding device <b>2132</b> are removed from rotatable unit <b>2114</b>. Guiding dispersion unit <b>2160</b> is mounted to the base <b>2104</b>, and substantially parallel with, dispersion unit <b>2110</b>, and is oriented towards dispersion unit <b>2110</b> to intercept reflected diffraction beam <b>2120</b>. Beam collector <b>2126</b>, optical coupling device <b>2130</b> and waveguiding device <b>2132</b> are mounted above dispersion unit <b>2110</b> and are fixed with respect to guiding dispersion unit <b>2160</b>.

In operation, the embodiment shown in FIG. 21 functions substantially the same as the embodiment of FIG. <b>14</b>. Reflected diffraction beam <b>2120</b> is substantially identical with reflected diffraction beam <b>1420</b> from FIG. <b>14</b>. Unlike in the embodiment of FIG. 14, however, reflected diffraction beam <b>2120</b> is not reflected by guiding mirror <b>1422</b>, but is instead diffracted by guiding dispersion unit <b>2160</b>.

Guiding dispersion unit <b>2160</b> is designed to diffract reflected diffraction beam <b>2120</b> in a pattern substantially identical with the pattern experienced by the light reflected by guiding mirror <b>1422</b> in the embodiment of FIG. <b>14</b>. Consequently, beam collector incident light <b>2124</b> is substantially identical with beam collector incident light <b>1424</b> from FIG. <b>14</b>. As a result, beam collector <b>2126</b> refracts and focuses beam collector incident light <b>2124</b> into focused light spot <b>2128</b> which is substantially identical to focused light spot <b>1428</b> of FIG. <b>14</b>.

Reflected diffraction beam <b>2124</b> comprised in collector incident radiation <b>2124</b> forms an angle χ(λ_L) with the normal of guiding dispersion unit <b>2160</b>, where

<maths><formula-text>χ(λ_L)=θ₀.</formula-text></maths>

As indicated by this formula, angle χ(λ_L) does not exhibit any dependence on wavelength or on the position of rotatable unit <b>2114</b>, but is instead fully determined by initial selection and alignment of the components of dual-beam laser system <b>2100</b>. To ensure full coupling of focused light spot <b>2128</b> into optical coupling device <b>2130</b>, the area of optical coupling device <b>2130</b> must be larger than the effective size of focused light spot <b>2128</b> and the numerical aperture of optical coupling device <b>2130</b> must be larger than the convergence of focused light spot <b>2128</b>. If these conditions are satisfied, proper initial design of dual-beam laser system <b>2100</b> results in continuous and stable coupling of selected focused light spot <b>2128</b> into waveguiding device <b>2132</b> with simultaneous and effective filtering of SSE and ASE background light regardless of wavelength variations in the system as a result of laser tuning.

FIG. 22 shows an alternative embodiment of the present invention. Dual-beam laser system <b>2200</b> comprises pivot <b>2202</b>, base <b>2204</b>, plane reflector <b>2206</b>, gain medium <b>2208</b>, dispersion unit <b>2210</b>, tuning reflector <b>2212</b>, rotatable unit <b>2214</b>, output laser beam <b>2216</b>, first-order diffracted radiation <b>2218</b>, reflected diffraction beam <b>2220</b>, beam collector incident light <b>2224</b>, beam collector <b>2226</b>, focused light spot <b>2228</b>, optical coupling device <b>2230</b>, waveguiding device <b>2232</b>, low noise laser beam <b>2234</b>, angularly-separated SSE <b>2236</b>, angulary-separated ASE <b>2238</b>, laser diode <b>2250</b>, collimation lens <b>2252</b> and guiding dispersion unit <b>2260</b>.

The structure of the embodiment shown in FIG. 22 is substantially identical with the structure of the embodiment shown in FIG. 21 except that the embodiment of FIG. 22 employs a laser diode <b>2250</b> to replace the combination of plane reflector <b>2106</b> and gain medium <b>2108</b> from FIG. <b>21</b> and introduces a collimation lens <b>2252</b> disposed along an optical path between laser diode <b>2250</b> and dispersion unit <b>2210</b>. Since the output of laser diode <b>2250</b> generally exhibits an undesirable elliptically-divergent shape, collimation lens <b>2252</b> is employed to collimate the light incident on dispersion unit <b>2210</b> at a grazing angle. In a preferred embodiment, both facets of collimation lens <b>2252</b> are treated with an anti-reflection coating to reduce light feedback from internal reflection within collimation lens <b>2252</b>. Further, a facet of laser diode <b>2250</b> oriented towards dispersion unit <b>2210</b> is also treated with an anti-reflection coating to maximize power output of laser diode <b>2250</b>.

In operation, the embodiment of FIG. 22 functions substantially the same as the embodiment of FIG. 21 because the light beam generated by laser diode <b>2250</b> in conjunction with collimation lens <b>2252</b> is substantially identical with the light beam produced by gain element <b>2108</b> and plane reflector <b>2106</b>. Consequently, the light incident at a grazing angle on diffraction grid <b>2210</b> is substantially identical with the light incident at a grazing angle on diffraction grid <b>2110</b>, and therefore the description provided for the embodiment of FIG. 21 also applies to the embodiment of FIG. <b>22</b>.

FIG. 23 shows yet another embodiment of the present invention. The preceding description of various embodiments of this invention taught how an optical aperture coupled to a waveguiding device can be employed as a spatial narrow band-pass filter to suppress SSE and ASE background light with wavelengths other than a desired wavelength. The embodiment of FIG. 23 illustrates how an optical transmission pinhole can be employed as a narrow band-pass filter to either replace or supplement and enhance the filtering effect of an optical coupling device.

FIG. 23 shows a simplified representation of the complete laser system described in prior embodiments. Laser system <b>2300</b> includes light generator <b>2370</b>, pinhole incident light <b>2372</b>, pinhole <b>2374</b>, beam collector <b>2326</b>, focused light spot <b>2328</b>, optical coupling device <b>2330</b> and waveguiding device <b>2332</b>.

Light generator <b>2370</b> and optical coupling device <b>2330</b> are mounted at opposite ends of laser system <b>2300</b>. Referring to the embodiment of FIG. 14 for example, light generator <b>2370</b> could include a subsystem comprising plane reflector <b>1406</b>, gain medium <b>1408</b>, dispersion unit <b>1410</b>, tuning reflector <b>1412</b> and guiding mirror <b>1422</b>. Pinhole <b>2374</b> and beam collector <b>2326</b> are disposed along an optical path between light generator <b>2370</b> and optical coupling device <b>2330</b> such that optical aperture <b>2330</b> is located distally from light generator <b>2370</b> with respect to pinhole <b>2374</b>. In an alternative embodiment, beam collector <b>2326</b> could be replaced by a concave mirror, as disclosed in the embodiment shown in FIG. 19 for example. Optical aperture <b>2330</b> is operationally connected to waveguiding device <b>2332</b> to permit coupling of light.

In operation, light generator <b>2370</b> projects pinhole incident light <b>2372</b> towards pinhole <b>2374</b>. Pinhole <b>2374</b> includes a transparent area which permits part of pinhole incident light <b>2372</b> to propagate beyond pinhole <b>2374</b> and illuminate beam collector <b>2326</b>. Beam collector <b>2326</b> focuses the incident light radiation into focused light spot <b>2328</b> which is pre-aligned with optical coupling device <b>2330</b> to permit efficient light coupling into waveguiding device <b>2332</b>. Pinhole <b>2374</b> acts as a physical spatial narrow band-pass filter effectively suppressing SSE and ASE background light, and therefore provides a first-order filtering stage for light propagating towards beam collector <b>2326</b>. Consequently, the light incident on beam collector <b>2326</b> is already filtered prior to being concentrated into focused light spot <b>2328</b>. This advantage could be employed, among others, to relax the design constraints imposed on the optical characteristics of beam collector <b>2326</b> and the alignment requirements associated with narrow band-pass filtering as taught by the present invention. Further, the SSE and ASE radiation may be cut off by the spatial filter so that a broader optical coupling device <b>2330</b> is still adequate to couple the laser beam into waveguiding device <b>2332</b>. A larger receiving aperture decreases the probability that optical coupling device <b>2330</b> is damaged by the heat produced by the high optical energy being coupled into waiveguiding device <b>2332</b>.

It will be manifest that various additions, modifications and rearrangements of the features of the invention may be made without deviating from the spirit and scope of the underlying inventive concept. It is intended that the scope of the invention as defined by the appended claims and their equivalents cover all such additions, modifications, and rearrangements. The appended claims are not to be interpreted as including means-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase “means-for.” Expedient embodiments of the invention are differentiated by the appended claims.